Merge branch 'develop' into claude/string-view-convertible-keys-5663

Conflicts: - docs/mkdocs/docs/api/basic_json/count.md: kept the PR's string_view note on overload 2 and develop's new item 3 (deleted integral-key overload) - docs/mkdocs/docs/api/basic_json/find.md: same as count.md - docs/mkdocs/docs/api/basic_json/value.md: kept develop's integral-key note on item 1 and the PR's string_view note on item 2 Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Fix key types convertible to std::string_view breaking lookups
2026-10-01 04:00:31 +00:00 · 2026-09-30 20:34:35 +02:00 · 2026-09-29 23:31:37 +02:00
22 changed files with 1241 additions and 2618 deletions
@@ -1395,7 +1395,7 @@ THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR I
 - The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the [MIT License](https://opensource.org/licenses/MIT) (see above). Copyright &copy; 2009 [Florian Loitsch](https://florian.loitsch.com/)
 - The class contains a copy of [Hedley](https://nemequ.github.io/hedley/) from Evan Nemerson which is licensed as [CC0-1.0](https://creativecommons.org/publicdomain/zero/1.0/).
 - The class contains parts of [Google Abseil](https://github.com/abseil/abseil-cpp) which is licensed under the [Apache 2.0 License](https://opensource.org/licenses/Apache-2.0).
- The class contains an adapted version of the Eisel-Lemire algorithm, its table of powers of five, and its digit comparison for long numbers from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
+- The class contains an adapted version of the Eisel-Lemire algorithm and its table of powers of five from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors

 <img align="right" src="https://git.fsfe.org/reuse/reuse-ci/raw/branch/master/reuse-horizontal.png" alt="REUSE Software">

@@ -224,5 +224,7 @@ Strong exception safety: if an exception occurs, the original value stays intact

 1. Added in version 1.0.0.
 2. Added in version 1.0.0.
-3. Added in version 3.11.0.
+3. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept `std::string_view`-convertible keys, as
+   already supported by [`operator[]`](operator[].md), [`value`](value.md), [`find`](find.md), and other lookup
+   functions.
 4. Added in version 2.0.0.
@@ -119,7 +119,9 @@ Logarithmic in the size of the JSON object.
 ## Version history

 1. Added in version 3.11.0.
-2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
+2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0. Fixed in
+   version 3.13.0 to consistently accept `std::string_view`-convertible keys, as already supported by
+   [`operator[]`](operator[].md), [`at`](at.md), [`value`](value.md), and other lookup functions.
 3. Added in version 3.7.0.
 4. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
   causing undefined behavior at runtime.
@@ -84,6 +84,8 @@ Logarithmic in the size of the JSON object.
 ## Version history

 1. Added in version 3.11.0.
-2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0.
+2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0. Fixed in version 3.13.0 to
+   consistently accept `std::string_view`-convertible keys, as already supported by [`operator[]`](operator[].md),
+   [`at`](at.md), [`value`](value.md), and other lookup functions.
 3. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of
   causing undefined behavior at runtime.
@@ -213,5 +213,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
 1. Added in version 1.0.0. Added support for binary types in version 3.8.0.
 2. Added in version 1.0.0. Added support for binary types in version 3.8.0.
 3. Added in version 1.0.0.
-4. Added in version 3.11.0.
+4. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept `std::string_view`-convertible keys, as
+   already supported by [`operator[]`](operator[].md), [`at`](at.md), [`value`](value.md), and other lookup
+   functions.
 5. Added in version 1.0.0.
@@ -88,6 +88,8 @@ Logarithmic in the size of the JSON object.
 ## Version history

 1. Added in version 3.11.0.
-2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0.
+2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0. Fixed in version 3.13.0 to
+   consistently accept `std::string_view`-convertible keys, as already supported by [`operator[]`](operator[].md),
+   [`at`](at.md), [`value`](value.md), and other lookup functions.
 3. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
   causing undefined behavior at runtime.
@@ -23,10 +23,9 @@ type to use.
 ## Template parameters

 `NumberFloatType`
-:   the type to store floating-point numbers. The parser converts `#!cpp float`, `#!cpp double`, and a
-    `#!cpp long double` that is IEEE 754 binary64 itself and other `#!cpp long double` formats with
-    `#!cpp std::from_chars` or `#!cpp std::strtold`, and serialization falls back to `#!cpp std::snprintf`, so the
-    type must be `#!cpp float`, `#!cpp double`, or `#!cpp long double`. The
+:   the type to store floating-point numbers. Parsing and serialization are implemented in terms of
+    `#!cpp std::strtof`/`#!cpp std::strtod`/`#!cpp std::strtold` and `#!cpp std::snprintf`, so the type must be
+    `#!cpp float`, `#!cpp double`, or `#!cpp long double`. The
    [binary formats](../../features/binary_formats/index.md) additionally require `#!cpp float` or `#!cpp double`,
    because they have no encoding for `#!cpp long double`. See
    [Template Parameter Requirements](../../features/types/template_parameters.md#numberfloattype).
@@ -195,7 +195,9 @@ changes to any JSON value.
 1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version
   3.11.0. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time
   instead of causing undefined behavior at runtime.
-2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2.
+2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2. Fixed in version 3.13.0
+   to consistently accept `std::string_view`-convertible keys, as already supported by
+   [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md), and other lookup functions.
 3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0, including fixing an issue where resolving
   `ptr` through an array unexpectedly threw `out_of_range` instead of returning the resolved element (or
   `default_value`, as documented).
@@ -71,11 +71,10 @@ otherwise, it uses unsigned integer storage.

    - Numbers with a decimal digit or scientific notation are always stored as `#!c double`.
    - The number types can be changed, see [Template number types](#template-number-types). 
-    - The library converts integers and floating-point numbers itself, independent of the locale. Floating-point
-      numbers are correctly rounded (to nearest, ties to even). Only a `#!c long double` that is not IEEE 754 binary64
-      (e.g., the 80-bit x87 format) is converted with `#!cpp std::from_chars` where available, or with
-      [`std::strtold`](https://en.cppreference.com/w/cpp/string/byte/strtof), which gets the decimal point of the
-      current locale, also one longer than one byte (e.g., in `fa_IR.UTF-8`).
+    - As of version 3.9.1, the conversion is realized by
+      [`std::strtoull`](https://en.cppreference.com/w/cpp/string/byte/strtoul),
+      [`std::strtoll`](https://en.cppreference.com/w/cpp/string/byte/strtol), and
+      [`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof), respectively.

 !!! example "Examples"

@@ -86,10 +85,10 @@ otherwise, it uses unsigned integer storage.
 ### Number limits

 - Any 64-bit signed or unsigned integer can be stored without loss of precision.
- Numbers exceeding the limits of `#!c double` (i.e., numbers whose rounded value is not satisfying
+- Numbers exceeding the limits of `#!c double` (i.e., numbers that after conversion via
+[`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof) are not satisfying
 [`std::isfinite`](https://en.cppreference.com/w/cpp/numeric/math/isfinite) such as `#!c 1E400`) will throw exception
-[`json.exception.out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) during parsing. Numbers too
-small for `#!c double` (such as `#!c 1E-400`) become zero, with the sign of the number.
+[`json.exception.out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) during parsing.
 - Floating-point numbers are rounded to the next number representable as `double`. For instance
 `#!c 3.141592653589793238462643383279` is stored as [`0x400921fb54442d18`](https://float.exposed/0x400921fb54442d18).
 This is the same behavior as the code `#!c double x = 3.141592653589793238462643383279;`.
@@ -26,9 +26,8 @@ Requirements are split into two groups:
    diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside
    the library. Four violations are not caught at compile time at all:

-    - A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers
-      stored as a `#!cpp long double` that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer
-      hands the buffer to `#!cpp std::strtold`.
+    - A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers,
+      because the lexer hands the buffer to `#!cpp std::strtoull`/`#!cpp std::strtoll`/`#!cpp std::strtod`.
    - A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation,
      and [`get_allocator()`](../../api/basic_json/get_allocator.md) each use a different default-constructed instance.
    - The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion
@@ -536,10 +535,8 @@ therefore silently changes parse results rather than raising an error. See

 `NumberFloatType` must be one of `#!cpp float`, `#!cpp double`, or `#!cpp long double`:

- The [parser](../parsing/index.md) converts number literals to `#!cpp float`, `#!cpp double`, and a
-  `#!cpp long double` that is IEEE 754 binary64 itself; other `#!cpp long double` formats are converted with
-  `#!cpp std::from_chars` where available, or with `#!cpp std::strtold`. The library provides overloads for exactly
-  these three types.
+- The [parser](../parsing/index.md) converts number literals with `#!cpp std::strtof`, `#!cpp std::strtod`, or
+  `#!cpp std::strtold`; the library provides overloads for exactly these three types.
 - [`dump`](../../api/basic_json/dump.md) falls back to `#!cpp std::snprintf` with the `%g` and `%Lg` conversion
  specifiers, for which the library likewise provides only `#!cpp double` and `#!cpp long double` overloads
  (`#!cpp float` is promoted to `#!cpp double`).
@@ -20,4 +20,4 @@ The class contains a slightly modified version of the Grisu2 algorithm from Flor

 The class contains a copy of [Hedley](https://nemequ.github.io/hedley/) from Evan Nemerson which is licensed as [CC0-1.0](https://creativecommons.org/publicdomain/zero/1.0/).

-The class contains an adapted version of the Eisel-Lemire algorithm, its table of powers of five, and its digit comparison for long numbers from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
+The class contains an adapted version of the Eisel-Lemire algorithm and its table of powers of five from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
@@ -39,25 +39,6 @@ inline int count_leading_zeros(std::uint64_t x) noexcept
 #endif
 }

-/// number of trailing zero bits of x (x != 0)
-inline int count_trailing_zeros(std::uint64_t x) noexcept
-{
-#if defined(__GNUC__) || defined(__clang__)
-    return __builtin_ctzll(x);
-#else
-    int n = 0;
-    for (int shift = 32; shift != 0; shift >>= 1)
-    {
-        if ((x << (64 - shift)) == 0)
-        {
-            n += shift;
-            x >>= shift;
-        }
-    }
-    return n;
-#endif
-}
-
 /// the 128-bit product of two 64-bit numbers
 struct uint128_parts
 {
@@ -87,19 +68,14 @@ inline uint128_parts full_multiplication(std::uint64_t a, std::uint64_t b) noexc

 /// eight bytes as a little-endian word (compilers fold this into one load on
 /// little-endian targets)
-inline std::uint64_t read_eight_bytes(const unsigned char* b) noexcept
+inline std::uint64_t read_eight_bytes(const char* p) noexcept
 {
+    const auto* b = reinterpret_cast<const unsigned char*>(p); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
    return static_cast<std::uint64_t>(b[0]) | (static_cast<std::uint64_t>(b[1]) << 8u)
           | (static_cast<std::uint64_t>(b[2]) << 16u) | (static_cast<std::uint64_t>(b[3]) << 24u)
           | (static_cast<std::uint64_t>(b[4]) << 32u) | (static_cast<std::uint64_t>(b[5]) << 40u)
           | (static_cast<std::uint64_t>(b[6]) << 48u) | (static_cast<std::uint64_t>(b[7]) << 56u);
 }

-/// eight bytes as a little-endian word
-inline std::uint64_t read_eight_bytes(const char* p) noexcept
-{
-    return read_eight_bytes(reinterpret_cast<const unsigned char*>(p)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
-}
-
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
@@ -1059,11 +1059,9 @@ class lexer : public lexer_base<BasicJsonType>
            token_type::parse_error otherwise

    @note The scanner is independent of the current locale: token_buffer
-          always holds `.`. The conversion of float and double does not use
-          the locale either. Only the std::strtold fallback of
-          convert_number() for long double formats other than binary64
-          depends on it, and it looks up the decimal point right before
-          converting (see detail::convert_float_locale_aware()).
+          always holds `.`. Only the std::strtod fallback of convert_number()
+          depends on the locale, and it looks up the decimal point right
+          before converting (see detail::convert_float_locale_aware()).
    */
    token_type scan_number()  // lgtm [cpp/use-of-goto] `goto` is used in this function to implement the number-parsing state machine described above. By design, any finite input will eventually reach the "done" state or return token_type::parse_error. In each intermediate state, 1 byte of the input is appended to the token_buffer vector, and only the already initialized variables token_buffer, number_type, and error_message are manipulated.
    {
@@ -1076,7 +1074,7 @@ class lexer : public lexer_base<BasicJsonType>

        // offset just past the last mantissa byte in token_buffer (i.e. the
        // index of 'e'/'E', or the whole token when there is no exponent).
-        // convert_number() uses it to split the token; npos means
+        // convert_number() uses it to count significant digits; npos means
        // "not seen an exponent yet" and is resolved at scan_number_done
        std::size_t mantissa_end = std::string::npos;

@@ -1406,8 +1404,8 @@ scan_number_done:
    @param[in] mantissa_end  offset just past the last mantissa byte in
                             token_buffer (the index of 'e'/'E', or
                             token_buffer.size() when there is no exponent);
-                             with decimal_point_position, it locates the parts
-                             of a float token without scanning it again
+                             used to skip Clinger's fast path when it cannot
+                             possibly succeed - see detail::mantissa_fits_clinger()
    */
    token_type convert_number(token_type number_type, std::size_t mantissa_end)
    {
@@ -1476,11 +1474,10 @@ scan_number_done:
        }

        // this code is reached if we parse a floating-point number or if an
-        // integer conversion above overflowed. float and double (and long
-        // double where it is binary64) are converted by the library itself,
-        // correctly rounded and independent of the locale; other long double
-        // formats use std::from_chars when available, otherwise the
-        // locale-aware strtold.
+        // integer conversion above overflowed. Prefer std::from_chars
+        // (Eisel-Lemire, locale-independent, correctly rounded) when available;
+        // otherwise the exact Clinger fast path (double only); otherwise the
+        // locale-aware strtof/strtod/strtold.
        if (convert_float_fast(num_begin, num_end, decimal_point_position, mantissa_end, value_float))
        {
            return token_type::value_float;
@@ -12,7 +12,6 @@
 #include <cstdint> // uint64_t
 #include <cstring> // memcpy

-#include <nlohmann/detail/bit_ops.hpp>
 #include <nlohmann/detail/macro_scope.hpp>

 // Optional SIMD backend for bulk UTF-8 validation. This is an opt-in external
@@ -70,12 +69,18 @@ inline std::size_t find_string_special(const unsigned char* data, std::size_t n)
    std::size_t i = 0;
    for (; i + 8 <= n; i += 8)
    {
-        const std::uint64_t special = swar_string_special(read_eight_bytes(data + i));
-        if (special != 0)
+        std::uint64_t word = 0;
+        std::memcpy(&word, data + i, sizeof(word));
+        if (swar_string_special(word) != 0)
        {
-            // the lowest flagged byte is the first special one: the borrows of
-            // the subtractions can only flag bytes above a true hit
-            return i + (static_cast<std::size_t>(count_trailing_zeros(special)) / 8);
+            // a special byte is in this word; locate it (endian-agnostic)
+            for (std::size_t j = 0; j < 8; ++j)
+            {
+                if (is_string_special(data[i + j]))
+                {
+                    return i + j;
+                }
+            }
        }
    }
    for (; i < n; ++i)
@@ -109,7 +114,8 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
    std::size_t i = 0;
    for (; i + 8 <= n; i += 8)
    {
-        const std::uint64_t v = read_eight_bytes(data + i);
+        std::uint64_t v = 0;
+        std::memcpy(&v, data + i, sizeof(v));
        const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'  (0x22)
        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\' (0x5C)
        const std::uint64_t d = v ^ 0x7F7F7F7F7F7F7F7Full; // DEL  (0x7F)
@@ -120,9 +126,7 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
                                   | (v & high);               // >= 0x80
        if (stop != 0)
        {
-            // the lowest flagged byte is the first one to stop at (see
-            // find_string_special())
-            return i + (static_cast<std::size_t>(count_trailing_zeros(stop)) / 8);
+            break;
        }
    }
    for (; i < n; ++i)
@@ -249,18 +253,12 @@ inline std::size_t scalar_string_bulk_run(const unsigned char* data, std::size_t
        {
            break; // end of buffer, or a quote/escape/control byte
        }
-        // a run of multi-byte sequences (e.g. CJK text) is validated sequence
-        // by sequence without searching for the next special byte in between
-        do
+        const std::size_t seq = validate_one_utf8(data + pos, n - pos);
+        if (seq == 0)
        {
-            const std::size_t seq = validate_one_utf8(data + pos, n - pos);
-            if (seq == 0)
-            {
-                return pos; // ill-formed or truncated: let the byte path diagnose it
-            }
-            pos += seq;
+            break; // ill-formed or truncated: let the byte path diagnose it
        }
-        while (pos < n && data[pos] >= 0x80u);
+        pos += seq;
    }
    return pos;
 }
@@ -275,7 +273,8 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
    std::size_t i = 0;
    for (; i + 8 <= n; i += 8)
    {
-        const std::uint64_t v = read_eight_bytes(data + i);
+        std::uint64_t v = 0;
+        std::memcpy(&v, data + i, sizeof(v));
        const std::uint64_t q = v ^ 0x2222222222222222ull;
        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull;
        const std::uint64_t hit = ((q - ones) & ~q & high)
@@ -283,8 +282,14 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
                                  | ((v - 0x2020202020202020ull) & ~v & high);
        if (hit != 0)
        {
-            // the lowest flagged byte is the first delimiter (see find_string_special())
-            return i + (static_cast<std::size_t>(count_trailing_zeros(hit)) / 8);
+            for (std::size_t j = 0; j < 8; ++j)
+            {
+                const unsigned char c = data[i + j];
+                if (c == '\"' || c == '\\' || c < 0x20u)
+                {
+                    return i + j;
+                }
+            }
        }
    }
    for (; i < n; ++i)
@@ -748,6 +748,30 @@ using is_usable_as_key_type = typename std::conditional <
                              std::true_type,
                              std::false_type >::type;

+#ifdef JSON_HAS_CPP_17
+// type trait to check if KeyType can only be used as an object key after
+// converting it to std::string_view: it is convertible to std::string_view, the
+// object's comparator cannot compare it with object_t::key_type directly, but
+// can compare a std::string_view. JSON pointers and JSON iterators are ruled out
+// first, so that the conversion checks are never instantiated for them (a JSON
+// pointer's deprecated conversion to string_t would be named otherwise).
+template < typename BasicJsonType, typename KeyTypeCVRef, typename KeyType = uncvref_t<KeyTypeCVRef>,
+           bool = is_json_pointer<KeyType>::value || is_json_iterator_of<BasicJsonType, KeyType>::value >
+struct is_string_view_convertible_key_type : std::false_type {};
+
+template<typename BasicJsonType, typename KeyTypeCVRef, typename KeyType>
+struct is_string_view_convertible_key_type<BasicJsonType, KeyTypeCVRef, KeyType, false>
+    : std::integral_constant < bool,
+      std::is_convertible<KeyTypeCVRef, std::string_view>::value
+      && !is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
+      typename BasicJsonType::object_t::key_type, KeyTypeCVRef, true, false>::value
+      && is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
+      typename BasicJsonType::object_t::key_type, std::string_view, true, false>::value > {};
+#else
+template<typename BasicJsonType, typename KeyTypeCVRef>
+struct is_string_view_convertible_key_type : std::false_type {};
+#endif
+
 // type trait to check if KeyType can be used as an object key
 // true if:
 //   - KeyType is comparable with BasicJsonType::object_t::key_type
@@ -761,9 +785,7 @@ using is_usable_as_basic_json_key_type = typename std::conditional <
     typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
     RequireTransparentComparator, ExcludeObjectKeyType>::value
     && !is_json_iterator_of<BasicJsonType, KeyType>::value)
-#ifdef JSON_HAS_CPP_17
-    || std::is_convertible<KeyType, std::string_view>::value
-#endif
+    || is_string_view_convertible_key_type<BasicJsonType, KeyTypeCVRef>::value
    , std::true_type,
    std::false_type >::type;

@@ -807,6 +807,24 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
        return it;
    }

+    /// @brief the key to look up an object member with: the key itself, or its
+    ///        std::string_view if the object can only be searched with that
+    template < typename KeyType, detail::enable_if_t <
+                   !detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value, int > = 0 >
+    static KeyType && lookup_key(KeyType && key) noexcept
+    {
+        return std::forward<KeyType>(key);
+    }
+
+#ifdef JSON_HAS_CPP_17
+    template < typename KeyType, detail::enable_if_t <
+                   detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value, int > = 0 >
+    static std::string_view lookup_key(KeyType && key)
+    {
+        return std::forward<KeyType>(key);
+    }
+#endif
+
    /// @brief erase an element from the object and return the following one
    /// Not every map returns an iterator from erase(iterator): some containers
    /// (e.g., Abseil's hash maps) return void to avoid computing a successor
@@ -2767,7 +2785,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
            JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
        }

-        auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
+        auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
        if (it == m_data.m_value.object->end())
        {
            JSON_THROW(out_of_range::create(403, detail::concat("key '", string_t(std::forward<KeyType>(key)), "' not found"), this));
@@ -2805,7 +2823,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
            JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
        }

-        auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
+        auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
        if (it == m_data.m_value.object->end())
        {
            JSON_THROW(out_of_range::create(403, detail::concat("key '", string_t(std::forward<KeyType>(key)), "' not found"), this));
@@ -2948,7 +2966,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
        // operator[] only works for objects
        if (JSON_HEDLEY_LIKELY(is_object()))
        {
-            auto result = m_data.m_value.object->emplace(std::forward<KeyType>(key), nullptr);
+            auto result = m_data.m_value.object->emplace(lookup_key(std::forward<KeyType>(key)), nullptr);
            return set_parent(result.first->second);
        }

@@ -2964,7 +2982,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
        // const operator[] only works for objects
        if (JSON_HEDLEY_LIKELY(is_object()))
        {
-            auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
+            auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
            JSON_ASSERT(it != m_data.m_value.object->end());
            return it->second;
        }
@@ -2974,8 +2992,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

  private:
    template<typename KeyType>
-    using is_comparable_with_object_key = detail::is_comparable <
-        object_comparator_t, const typename object_t::key_type&, KeyType >;
+    using is_comparable_with_object_key = std::integral_constant < bool,
+        detail::is_comparable <
+            object_comparator_t, const typename object_t::key_type&, KeyType >::value
+        || detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value >;

    template<typename ValueType>
    using value_return_type = std::conditional <
@@ -3362,7 +3382,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
            JSON_THROW(type_error::create(307, detail::concat("cannot use erase() with ", type_name()), this));
        }

-        const auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
+        const auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
        if (it != m_data.m_value.object->end())
        {
            m_data.m_value.object->erase(it);
@@ -3389,7 +3409,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
    size_type erase(KeyType && key)
    {
-        return erase_internal(std::forward<KeyType>(key));
+        return erase_internal(lookup_key(std::forward<KeyType>(key)));
    }

    /// @brief remove element from a JSON array given an index
@@ -3469,7 +3489,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

        if (is_object())
        {
-            result.m_it.object_iterator = m_data.m_value.object->find(std::forward<KeyType>(key));
+            result.m_it.object_iterator = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
        }

        return result;
@@ -3485,7 +3505,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

        if (is_object())
        {
-            result.m_it.object_iterator = m_data.m_value.object->find(std::forward<KeyType>(key));
+            result.m_it.object_iterator = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
        }

        return result;
@@ -3508,7 +3528,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
    size_type count(KeyType && key) const
    {
        // return 0 for all nonobject types
-        return is_object() ? m_data.m_value.object->count(std::forward<KeyType>(key)) : 0;
+        return is_object() ? m_data.m_value.object->count(lookup_key(std::forward<KeyType>(key))) : 0;
    }

    /// @brief check the existence of an element in a JSON object
@@ -3526,7 +3546,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
    JSON_HEDLEY_WARN_UNUSED_RESULT
    bool contains(KeyType && key) const
    {
-        return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
+        return is_object() && m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key))) != m_data.m_value.object->end();
    }

    /// @brief check the existence of an element in a JSON object given a JSON pointer
@@ -1,599 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
-// |  |  |__   |  |  | | | |  version 3.12.0
-// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
-//
-// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
-// SPDX-License-Identifier: MIT
-
-#pragma once
-
-#include <array> // array
-#include <cstdint> // uint32_t, uint64_t
-
-// Number tokens that are hard to round correctly, with the IEEE-754 binary64
-// and binary32 bits of their correctly rounded values (ties to even; infinity
-// for an overflow, a signed zero for an underflow).
-//
-// For doubles and floats around 0, the smallest normal number, 1, 2^24, 2^53,
-// 0.1, and the largest finite number, and for random ones, the exact midpoint
-// m to the next number gives: m, m with one unit more and less in the last
-// digit, m with "01" and "0...01" appended, m with trailing zeros, and m cut
-// after 17 to 30 digits (rounded down and up, so that the rounding is decided
-// after the 19th digit), in fixed and exponent notation, 30% of them negative.
-// Tokens longer than 80 characters are left out, except for four of 700 digits
-// and more. Zeros, underflow, overflow, huge exponents, and integers beyond 64
-// bits complete the set. Of the 508 tokens, 134 (as double) and 150 (as
-// float) need the exact comparison with the midpoint (detail::digit_comparison()).
-//
-// The expected bits were computed with exact rational arithmetic in Python
-// (fractions.Fraction) and cross-checked with Python's float(); strtod_l and
-// strtof_l of Apple's libc and of glibc agree. Generated by
-// compact_hard_cases.py 5 (with hard_cases.py), see the pull request that
-// added this file.
-
-namespace float_hard_cases
-{
-
-struct hard_case
-{
-    const char* token;
-    std::uint64_t bits64;
-    std::uint32_t bits32;
-};
-
-inline const std::array<hard_case, 508>& cases()
-{
-    static const std::array<hard_case, 508> table =
-    {
-        {
-            {"-2.4703282292062327e-324", 0x8000000000000000u, 0x80000000u},
-            {"24703282292062328e-340", 0x0000000000000001u, 0x00000000u},
-            {"247032822920623272e-341", 0x0000000000000000u, 0x00000000u},
-            {"-0.2470328229206232721e-323", 0x8000000000000001u, 0x80000000u},
-            {"-0.24703282292062327208e-323", 0x8000000000000000u, 0x80000000u},
-            {"-2.4703282292062327209e-324", 0x8000000000000001u, 0x80000000u},
-            {"2.47032822920623272088e-324", 0x0000000000000000u, 0x00000000u},
-            {"247032822920623272089e-344", 0x0000000000000001u, 0x00000000u},
-            {"-247032822920623272088284396434e-353", 0x8000000000000000u, 0x80000000u},
-            {"0.247032822920623272088284396435e-323", 0x0000000000000001u, 0x00000000u},
-            {"-74109846876186981e-340", 0x8000000000000001u, 0x80000000u},
-            {"0.74109846876186982e-323", 0x0000000000000002u, 0x00000000u},
-            {"-0.7410984687618698162e-323", 0x8000000000000001u, 0x80000000u},
-            {"-7.410984687618698163e-324", 0x8000000000000002u, 0x80000000u},
-            {"7.4109846876186981626e-324", 0x0000000000000001u, 0x00000000u},
-            {"-74109846876186981627e-343", 0x8000000000000002u, 0x80000000u},
-            {"-741098468761869816264e-344", 0x8000000000000001u, 0x80000000u},
-            {"0.741098468761869816265e-323", 0x0000000000000002u, 0x00000000u},
-            {"0.741098468761869816264853189302e-323", 0x0000000000000001u, 0x00000000u},
-            {"-7.41098468761869816264853189303e-324", 0x8000000000000002u, 0x80000000u},
-            {"0.22250738585072006e-307", 0x000FFFFFFFFFFFFEu, 0x00000000u},
-            {"2.2250738585072007e-308", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"2.225073858507200641e-308", 0x000FFFFFFFFFFFFEu, 0x00000000u},
-            {"-2225073858507200642e-326", 0x800FFFFFFFFFFFFFu, 0x80000000u},
-            {"22250738585072006419e-327", 0x000FFFFFFFFFFFFEu, 0x00000000u},
-            {"0.2225073858507200642e-307", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"0.222507385850720064199e-307", 0x000FFFFFFFFFFFFEu, 0x00000000u},
-            {"2.225073858507200642e-308", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"-2.22507385850720064199176395546e-308", 0x800FFFFFFFFFFFFEu, 0x80000000u},
-            {"222507385850720064199176395547e-337", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"-2.2250738585072011e-308", 0x800FFFFFFFFFFFFFu, 0x80000000u},
-            {"-22250738585072012e-324", 0x8010000000000000u, 0x80000000u},
-            {"-2225073858507201136e-326", 0x800FFFFFFFFFFFFFu, 0x80000000u},
-            {"0.2225073858507201137e-307", 0x0010000000000000u, 0x00000000u},
-            {"0.2225073858507201136e-307", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"-2.2250738585072011361e-308", 0x8010000000000000u, 0x80000000u},
-            {"2.22507385850720113605e-308", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"222507385850720113606e-328", 0x0010000000000000u, 0x00000000u},
-            {"22250738585072011360574097967e-336", 0x000FFFFFFFFFFFFFu, 0x00000000u},
-            {"0.222507385850720113605740979671e-307", 0x0010000000000000u, 0x00000000u},
-            {"22250738585072016e-324", 0x0010000000000000u, 0x00000000u},
-            {"0.22250738585072017e-307", 0x0010000000000001u, 0x00000000u},
-            {"0.222507385850720163e-307", 0x0010000000000000u, 0x00000000u},
-            {"2.225073858507201631e-308", 0x0010000000000001u, 0x00000000u},
-            {"-2.2250738585072016301e-308", 0x8010000000000000u, 0x80000000u},
-            {"22250738585072016302e-327", 0x0010000000000001u, 0x00000000u},
-            {"-222507385850720163012e-328", 0x8010000000000000u, 0x80000000u},
-            {"0.222507385850720163013e-307", 0x0010000000000001u, 0x00000000u},
-            {"0.222507385850720163012305563795e-307", 0x0010000000000000u, 0x00000000u},
-            {"-2.22507385850720163012305563796e-308", 0x8010000000000001u, 0x80000000u},
-            {"0.17976931348623156E+309", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
-            {"1.7976931348623157e308", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"1.797693134862315608e308", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
-            {"-1797693134862315609e290", 0xFFEFFFFFFFFFFFFFu, 0xFF800000u},
-            {"-17976931348623156083e289", 0xFFEFFFFFFFFFFFFEu, 0xFF800000u},
-            {"-0.17976931348623156084E+309", 0xFFEFFFFFFFFFFFFFu, 0xFF800000u},
-            {"0.179769313486231560835E+309", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
-            {"-1.79769313486231560836e308", 0xFFEFFFFFFFFFFFFFu, 0xFF800000u},
-            {"1.79769313486231560835325876058e308", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
-            {"179769313486231560835325876059e279", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"1.7976931348623158e308", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"17976931348623159e292", 0x7FF0000000000000u, 0x7F800000u},
-            {"1797693134862315807e290", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"0.1797693134862315808E+309", 0x7FF0000000000000u, 0x7F800000u},
-            {"0.17976931348623158079E+309", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"-1.797693134862315808e308", 0xFFF0000000000000u, 0xFF800000u},
-            {"1.79769313486231580793e308", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"179769313486231580794e288", 0x7FF0000000000000u, 0x7F800000u},
-            {"179769313486231580793728971405e279", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"-0.179769313486231580793728971406E+309", 0xFFF0000000000000u, 0xFF800000u},
-            {"100000000000000011102230246251565404236316680908203125e-53", 0x3FF0000000000000u, 0x3F800000u},
-            {"-1.00000000000000011102230246251565404236316680908203126", 0xBFF0000000000001u, 0xBF800000u},
-            {"1.00000000000000011102230246251565404236316680908203124e0", 0x3FF0000000000000u, 0x3F800000u},
-            {"10000000000000001110223024625156540423631668090820312501e-55", 0x3FF0000000000001u, 0x3F800000u},
-            {"1.00000000000000011102230246251565404236316680908203125000000000000000000001", 0x3FF0000000000001u, 0x3F800000u},
-            {"10000000000000001e-16", 0x3FF0000000000000u, 0x3F800000u},
-            {"1.0000000000000002", 0x3FF0000000000001u, 0x3F800000u},
-            {"1.000000000000000111", 0x3FF0000000000000u, 0x3F800000u},
-            {"1.000000000000000112e0", 0x3FF0000000000001u, 0x3F800000u},
-            {"1.000000000000000111e0", 0x3FF0000000000000u, 0x3F800000u},
-            {"-10000000000000001111e-19", 0xBFF0000000000001u, 0xBF800000u},
-            {"-100000000000000011102e-20", 0xBFF0000000000000u, 0xBF800000u},
-            {"-1.00000000000000011103", 0xBFF0000000000001u, 0xBF800000u},
-            {"1.00000000000000011102230246251", 0x3FF0000000000000u, 0x3F800000u},
-            {"1.00000000000000011102230246252e0", 0x3FF0000000000001u, 0x3F800000u},
-            {"-0.999999999999999944488848768742172978818416595458984375", 0xBFF0000000000000u, 0xBF800000u},
-            {"-9.99999999999999944488848768742172978818416595458984376e-1", 0xBFF0000000000000u, 0xBF800000u},
-            {"999999999999999944488848768742172978818416595458984374e-54", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"0.99999999999999994448884876874217297881841659545898437501", 0x3FF0000000000000u, 0x3F800000u},
-            {"9.99999999999999944488848768742172978818416595458984375000000000000000000001e-1", 0x3FF0000000000000u, 0x3F800000u},
-            {"-0.99999999999999994", 0xBFEFFFFFFFFFFFFFu, 0xBF800000u},
-            {"9.9999999999999995e-1", 0x3FF0000000000000u, 0x3F800000u},
-            {"9.999999999999999444e-1", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"9999999999999999445e-19", 0x3FF0000000000000u, 0x3F800000u},
-            {"99999999999999994448e-20", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"-0.99999999999999994449", 0xBFF0000000000000u, 0xBF800000u},
-            {"0.999999999999999944488", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"-9.99999999999999944489e-1", 0xBFF0000000000000u, 0xBF800000u},
-            {"9.99999999999999944488848768742e-1", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"999999999999999944488848768743e-30", 0x3FF0000000000000u, 0x3F800000u},
-            {"-9.007199254740993e15", 0xC340000000000000u, 0xDA000000u},
-            {"9007199254740994e0", 0x4340000000000001u, 0x5A000000u},
-            {"9007199254740992", 0x4340000000000000u, 0x5A000000u},
-            {"9.00719925474099301e15", 0x4340000000000001u, 0x5A000000u},
-            {"9007199254740993000000000000000000001e-21", 0x4340000000000001u, 0x5A000000u},
-            {"-9007199254740993.000000000000000000000000000000", 0xC340000000000000u, 0xDA000000u},
-            {"90071992547409915e-1", 0x4340000000000000u, 0x5A000000u},
-            {"-9007199254740991.6", 0xC340000000000000u, 0xDA000000u},
-            {"9.0071992547409914e15", 0x433FFFFFFFFFFFFFu, 0x5A000000u},
-            {"-9007199254740991501e-3", 0xC340000000000000u, 0xDA000000u},
-            {"9007199254740991.5000000000000000000001", 0x4340000000000000u, 0x5A000000u},
-            {"9.0071992547409915000000000000000000000000000000e15", 0x4340000000000000u, 0x5A000000u},
-            {"0.100000000000000012490009027033011079765856266021728515625", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"1.00000000000000012490009027033011079765856266021728515626e-1", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"100000000000000012490009027033011079765856266021728515624e-57", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"0.10000000000000001249000902703301107976585626602172851562501", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"0.10000000000000001", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"1.0000000000000002e-1", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"-1.000000000000000124e-1", 0xBFB999999999999Au, 0xBDCCCCCDu},
-            {"1000000000000000125e-19", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"-10000000000000001249e-20", 0xBFB999999999999Au, 0xBDCCCCCDu},
-            {"0.1000000000000000125", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"0.10000000000000001249", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"1.00000000000000012491e-1", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"1.00000000000000012490009027033e-1", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"100000000000000012490009027034e-30", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"2.45134755833537796875e14", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"-245134755833537796876e-6", 0xC2EBDE5C4164D83Au, 0xD75EF2E2u},
-            {"-245134755833537.796874", 0xC2EBDE5C4164D839u, 0xD75EF2E2u},
-            {"2.4513475583353779687501e14", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"245134755833537796875000000000000000000001e-27", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"245134755833537.796875000000000000000000000000000000", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"2.4513475583353779e14", 0x42EBDE5C4164D839u, 0x575EF2E2u},
-            {"2451347558335378e-1", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"2451347558335377968e-4", 0x42EBDE5C4164D839u, 0x575EF2E2u},
-            {"245134755833537.7969", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"245134755833537.79687", 0x42EBDE5C4164D839u, 0x575EF2E2u},
-            {"2.4513475583353779688e14", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"181510327827821147441864013671875e-23", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"-1815103278.27821147441864013671876", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
-            {"1.81510327827821147441864013671874e9", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"18151032782782114744186401367187501e-25", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"1815103278.27821147441864013671875000000000000000000001", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"-1.81510327827821147441864013671875000000000000000000000000000000e9", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
-            {"18151032782782114e-7", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"1815103278.2782115", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"1815103278.278211474", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"-1.815103278278211475e9", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
-            {"1.8151032782782114744e9", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"-18151032782782114745e-10", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
-            {"181510327827821147441e-11", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"1815103278.27821147442", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"1815103278.27821147441864013671", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"1.81510327827821147441864013672e9", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"3809325632181785344", 0x43CA6EB8BD69FE2Au, 0x5E5375C6u},
-            {"3.809325632181785345e18", 0x43CA6EB8BD69FE2Au, 0x5E5375C6u},
-            {"3809325632181785343e0", 0x43CA6EB8BD69FE29u, 0x5E5375C6u},
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-            {"-92331620", 0xC196037990000000u, 0xCCB01BCCu},
-            {"9.233163e7", 0x41960379B8000000u, 0x4CB01BCEu},
-            {"9233161e1", 0x4196037968000000u, 0x4CB01BCBu},
-            {"92331620.1", 0x4196037990666666u, 0x4CB01BCDu},
-            {"9.233162000000000000000000001e7", 0x4196037990000000u, 0x4CB01BCDu},
-            {"9233162000000000000000000000000000000e-29", 0x4196037990000000u, 0x4CB01BCCu},
-            {"3.002458625e6", 0x4146E82D50000000u, 0x4A37416Au},
-            {"3002458626e-3", 0x4146E82D5020C49Cu, 0x4A37416Bu},
-            {"3002458.624", 0x4146E82D4FDF3B64u, 0x4A37416Au},
-            {"-3.00245862501e6", 0xC146E82D500053E3u, 0xCA37416Bu},
-            {"3002458625000000000000000000001e-24", 0x4146E82D50000000u, 0x4A37416Bu},
-            {"3002458.625000000000000000000000000000000", 0x4146E82D50000000u, 0x4A37416Au},
-            {"-1095485584696182596504479582065262592e1", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"10954855846961825965044795820652625930", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"1.095485584696182596504479582065262591e37", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"109548558469618259650447958206526259201e-1", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"-10954855846961825965044795820652625920.00000000000000000001", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"1.095485584696182596504479582065262592000000000000000000000000000000e37", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"10954855846961825e21", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"10954855846961826000000000000000000000", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"10954855846961825960000000000000000000", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"1.095485584696182597e37", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"1.0954855846961825965e37", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"-10954855846961825966e18", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"10954855846961825965e18", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"10954855846961825965100000000000000000", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"10954855846961825965044795820600000000", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"-1.09548558469618259650447958207e37", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"1.6449216019182103706535606608388384863861375606575165875256061553955078126e-21", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"-16449216019182103706535606608388384863861375606575165875256061553955078124e-94", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"-0.0000000000000000000016449216019182103", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"-1.6449216019182104e-21", 0xBB9F125A50000000u, 0x9CF892D3u},
-            {"-1.64492160191821037e-21", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"-1644921601918210371e-39", 0xBB9F125A50000000u, 0x9CF892D3u},
-            {"-16449216019182103706e-40", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"0.0000000000000000000016449216019182103707", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"0.00000000000000000000164492160191821037065", 0x3B9F125A50000000u, 0x1CF892D2u},
-            {"1.64492160191821037066e-21", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"1.64492160191821037065356066083e-21", 0x3B9F125A50000000u, 0x1CF892D2u},
-            {"164492160191821037065356066084e-50", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"6.565061509609222412109375e-1", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"6565061509609222412109376e-25", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"0.6565061509609222412109374", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"6.56506150960922241210937501e-1", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"6565061509609222412109375000000000000000000001e-46", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"-0.6565061509609222412109375000000000000000000000000000000", 0xBFE5021930000000u, 0xBF2810CAu},
-            {"6.5650615096092224e-1", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"-65650615096092225e-17", 0xBFE5021930000000u, 0xBF2810CAu},
-            {"6565061509609222412e-19", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"0.6565061509609222413", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"0.65650615096092224121", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"6.5650615096092224122e-1", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"-6.5650615096092224121e-1", 0xBFE5021930000000u, 0xBF2810C9u},
-            {"656506150960922241211e-21", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"18014627239033005156980393746124491372029297053813934326171875e-77", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"0.00000000000000018014627239033005156980393746124491372029297053813934326171876", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"-1.8014627239033005156980393746124491372029297053813934326171874e-16", 0xBCA9F63970000000u, 0xA54FB1CBu},
-            {"1801462723903300515698039374612449137202929705381393432617187501e-79", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"18014627239033005e-32", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"0.00000000000000018014627239033006", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"0.0000000000000001801462723903300515", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"-1.801462723903300516e-16", 0xBCA9F63970000000u, 0xA54FB1CCu},
-            {"-1.8014627239033005156e-16", 0xBCA9F63970000000u, 0xA54FB1CBu},
-            {"18014627239033005157e-35", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"180146272390330051569e-36", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"-0.00000000000000018014627239033005157", 0xBCA9F63970000000u, 0xA54FB1CCu},
-            {"0.000000000000000180146272390330051569803937461", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"1.80146272390330051569803937462e-16", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"0.05534819327294826507568359375", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"-5.534819327294826507568359376e-2", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"5534819327294826507568359374e-29", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"-0.0553481932729482650756835937501", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"5.534819327294826507568359375000000000000000000001e-2", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"5534819327294826507568359375000000000000000000000000000000e-59", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"0.055348193272948265", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"5.5348193272948266e-2", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"5.534819327294826507e-2", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"5534819327294826508e-20", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"55348193272948265075e-21", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"-0.055348193272948265076", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"0.0553481932729482650756", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"-5.53481932729482650757e-2", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"5.179692133247783258005389047985340416e36", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"5179692133247783258005389047985340417e0", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"5179692133247783258005389047985340415", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"-5.17969213324778325800538904798534041601e36", 0xC78F2C9450000000u, 0xFC7964A3u},
-            {"-5179692133247783258005389047985340416000000000000000000001e-21", 0xC78F2C9450000000u, 0xFC7964A3u},
-            {"-5179692133247783258005389047985340416.000000000000000000000000000000", 0xC78F2C9450000000u, 0xFC7964A2u},
-            {"5.1796921332477832e36", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"51796921332477833e20", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"-5179692133247783258e18", 0xC78F2C9450000000u, 0xFC7964A2u},
-            {"5179692133247783259000000000000000000", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"5179692133247783258000000000000000000", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"5.1796921332477832581e36", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"-5.179692133247783258e36", 0xC78F2C9450000000u, 0xFC7964A2u},
-            {"-517969213324778325801e16", 0xC78F2C9450000000u, 0xFC7964A3u},
-            {"517969213324778325800538904798e7", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"5179692133247783258005389047990000000", 0x478F2C9450000000u, 0x7C7964A3u},
-            {
-                "0.22250738585072011360574097967091319759348195463516456480234261097248222220210769455165295239081350"
-                "8791414915891303962110687008643869459464552765720740782062174337998814106326732925355228688137214901"
-                "2981122451451889849057222307285255133155755015914397476397983411801999323962548289017107081850690630"
-                "6666559949382757725720157630626906633326475653000092458883164330377797918696120494973903778297049050"
-                "5108060994073026293712895895000358379996720725430436028407889577179615094551674824347103070260914462"
-                "1572289880258182545180325707018860872113128079512233426288368622321503775666622503982534335974568884"
-                "4239002654981983854879482922068947216898310996983658468140228542433306603398508864458040010349339704"
-                "2756718644338377048603786162277173854562306587467901408672332763671875e-307", 0x0010000000000000u, 0x00000000u
-            },
-            {
-                "2.22507385850720113605740979670913197593481954635164564802342610972482222202107694551652952390813508"
-                "7914149158913039621106870086438694594645527657207407820621743379988141063267329253552286881372149012"
-                "9811224514518898490572223072852551331557550159143974763979834118019993239625482890171070818506906306"
-                "6665599493827577257201576306269066333264756530000924588831643303777979186961204949739037782970490505"
-                "1080609940730262937128958950003583799967207254304360284078895771796150945516748243471030702609144621"
-                "5722898802581825451803257070188608721131280795122334262883686223215037756666225039825343359745688844"
-                "2390026549819838548794829220689472168983109969836584681402285424333066033985088644580400103493397042"
-                "756718644338377048603786162277173854562306587467901408672332763671875000000000000000000001e-308", 0x0010000000000000u, 0x00000000u
-            },
-            {
-                "0.11754942807573642917278829910357665133228589927589904276829631184250030649651730385585324256680905"
-                "8189392089843750000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "00000000000000000000000000000000000000000000000000000000000000000e-37", 0x380FFFFFE0000000u, 0x00800000u
-            },
-            {
-                "1175494280757364291727882991035766513322858992758990427682963118425003064965173038558532425668090581"
-                "8939208984375000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "00000000000001e-751", 0x380FFFFFE0000000u, 0x00800000u
-            },
-            {"0", 0x0000000000000000u, 0x00000000u},
-            {"-0", 0x8000000000000000u, 0x80000000u},
-            {"0.0", 0x0000000000000000u, 0x00000000u},
-            {"-0.0", 0x8000000000000000u, 0x80000000u},
-            {"0e999999999999999999999", 0x0000000000000000u, 0x00000000u},
-            {"-0.000e-99999", 0x8000000000000000u, 0x80000000u},
-            {"1e-400", 0x0000000000000000u, 0x00000000u},
-            {"-1e-400", 0x8000000000000000u, 0x80000000u},
-            {"1e400", 0x7FF0000000000000u, 0x7F800000u},
-            {"-1e400", 0xFFF0000000000000u, 0xFF800000u},
-            {"1e-50", 0x358DEE7A4AD4B81Fu, 0x00000000u},
-            {"-1e-50", 0xB58DEE7A4AD4B81Fu, 0x80000000u},
-            {"1e39", 0x48078287F49C4A1Du, 0x7F800000u},
-            {"-1e39", 0xC8078287F49C4A1Du, 0xFF800000u},
-            {"1e99999999999999999999999999", 0x7FF0000000000000u, 0x7F800000u},
-            {"1e-99999999999999999999999999", 0x0000000000000000u, 0x00000000u},
-            {"1e0000000000000000000000000000000000000000308", 0x7FE1CCF385EBC8A0u, 0x7F800000u},
-            {"123456789012345678901234567890e-30", 0x3FBF9ADD3746F65Fu, 0x3DFCD6EAu},
-            {"18446744073709551615", 0x43F0000000000000u, 0x5F800000u},
-            {"18446744073709551616", 0x43F0000000000000u, 0x5F800000u},
-            {"-9223372036854775808", 0xC3E0000000000000u, 0xDF000000u},
-            {"-9223372036854775809", 0xC3E0000000000000u, 0xDF000000u},
-        }
-    };
-    return table;
-}
-
-} // namespace float_hard_cases
@@ -13,18 +13,15 @@
 using nlohmann::json;

 #include <array> // array
+#include <cfloat> // FLT_EVAL_METHOD
 #include <cstdint> // uint32_t, uint64_t
-#include <cstdio> // snprintf
 #include <cstdlib> // strtod
 #include <cstring> // memcpy
-#include <map> // map
 #include <sstream> // stringstream
 #include <string> // string
 #include <utility> // pair
 #include <vector> // vector

-#include "float_hard_cases.hpp"
-
 namespace
 {
 // shortcut to scan a string literal
@@ -260,7 +257,7 @@ TEST_CASE("lexer number fast path")
            "123456789012345678901234567890",  // huge -> float
            "0.30000000000000004", "2.2250738585072014e-308", "1e308",
            // high-precision / wide-exponent values that exercise the
-            // Eisel-Lemire path beyond the Clinger subset
+            // std::from_chars (Eisel-Lemire) path beyond the Clinger subset
            "1.7976931348623157e308", "1.2345678901234567e-250",
            "9007199254740993", "5e-324", "1e-320"
        };
@@ -282,18 +279,20 @@ TEST_CASE("lexer number fast path")
        }
    }

-    SECTION("significant digits around Clinger's fast path")
+    SECTION("significant-digit gate for the Clinger fast path")
    {
-        // Clinger's fast path needs a significand of at most 2^53, which
-        // tokens with 17 or more significant digits exceed. The conversion
-        // splits the token at the positions the scanners recorded, so leading
-        // zeros must not count as digits - "0.1234567890123456" has 16
-        // significant digits, not 17 - and both scanners must agree.
+        // Clinger's fast path needs a significand below 2^53, so it cannot
+        // succeed once the mantissa has 17 or more significant digits (the
+        // significand would be at least 10^16). The lexer skips the attempt
+        // there. That is only allowed to save work: every value must still come
+        // out bit-exactly, and both scanners must agree. In particular the gate
+        // must not fire for tokens whose leading zeros merely look like extra
+        // digits - "0.1234567890123456" has 16 significant digits, not 17.
        const std::vector<std::string> numbers =
        {
            "1234567890123456",                  // 16 significant digits
-            "12345678901234567",                 // 17
-            "123456789012345678",                // 18
+            "12345678901234567",                 // 17 -> attempt skipped
+            "123456789012345678",                // 18 -> attempt skipped
            "0.1234567890123456",                // 16: the leading "0" is not significant
            "0.12345678901234567",               // 17
            "0.00000000000000001",               // 1, in a long token
@@ -664,145 +663,46 @@ TEST_CASE("lexer string fast path")
    }
 }

-namespace
+TEST_CASE("parse_float_fast declines what it cannot convert exactly")
 {
-// the index of the decimal point (or npos) and of the end of the mantissa of a
-// number token, which the lexer records while scanning it
-std::pair<std::size_t, std::size_t> float_token_layout(const std::string& s)
-{
-    std::size_t dot = std::string::npos;
-    std::size_t mantissa_end = s.size();
-    for (std::size_t i = 0; i < s.size(); ++i)
+    // The lexer only hands well-formed numbers to parse_float_fast, so the
+    // malformed ones below can only be passed to it directly. Declining is
+    // always safe: the caller then falls back to a slower, exact conversion.
+    const auto fast = [](const std::string & s, double & out)
    {
-        if (s[i] == '.')
-        {
-            dot = i;
-        }
-        else if (s[i] == 'e' || s[i] == 'E')
-        {
-            mantissa_end = i;
-            break;
-        }
-    }
-    return {dot, mantissa_end};
-}
+        return nlohmann::detail::parse_float_fast(s.data(), s.data() + s.size(), out);
+    };
+    double out = 0;

-template<typename FloatType>
-FloatType parse_native(const std::string& s)
-{
-    const auto layout = float_token_layout(s);
-    return nlohmann::detail::parse_float_native<FloatType>(s.data(), s.data() + s.size(), layout.first, layout.second);
-}
+#if defined(FLT_EVAL_METHOD) && FLT_EVAL_METHOD != 0
+    // without true double precision, the fast path declines everything
+    CHECK_FALSE(fast("1.5", out));
+#else
+    CHECK(fast("1.5", out));
+    CHECK(out == 1.5);
+    CHECK(fast("+2.5e1", out));
+    CHECK(out == 25.0);
+    CHECK(fast("-25E-1", out));
+    CHECK(out == -2.5);
+    CHECK(fast("1e", out));
+    CHECK(out == 1.0);
+#endif

-std::uint64_t bits_of(double d)
-{
-    std::uint64_t b = 0;
-    std::memcpy(&b, &d, sizeof(b));
-    return b;
-}
+    // not a number
+    CHECK_FALSE(fast("", out));
+    CHECK_FALSE(fast("-", out));
+    CHECK_FALSE(fast(".", out));
+    CHECK_FALSE(fast("1.2.3", out));
+    CHECK_FALSE(fast("1x", out));
+    CHECK_FALSE(fast("1e+", out));
+    CHECK_FALSE(fast("1e1x", out));

-std::uint32_t bits_of(float f)
-{
-    std::uint32_t b = 0;
-    std::memcpy(&b, &f, sizeof(b));
-    return b;
-}
-
-std::uint64_t native_bits64(const std::string& s)
-{
-    return bits_of(parse_native<double>(s));
-}
-
-std::uint32_t native_bits32(const std::string& s)
-{
-    return bits_of(parse_native<float>(s));
-}
-} // namespace
-
-TEST_CASE("parse_float_native rounds correctly")
-{
-    SECTION("double")
-    {
-        CHECK(native_bits64("1.5") == 0x3FF8000000000000u);
-        CHECK(native_bits64("0.1") == 0x3FB999999999999Au);
-        CHECK(native_bits64("-0.0") == 0x8000000000000000u);
-        CHECK(native_bits64("0e999999999999999999999") == 0u);
-        // 2^53 + 1 is exactly between two doubles: ties to even, unless more digits follow
-        CHECK(native_bits64("9007199254740993") == 0x4340000000000000u);
-        CHECK(native_bits64("9007199254740993.0000000000000000001") == 0x4340000000000001u);
-        CHECK(native_bits64("9007199254740992.9999999999999999999") == 0x4340000000000000u);
-        // 1 + 2^-53 exactly (a tie), and one unit in the 55th digit around it
-        CHECK(native_bits64("1.00000000000000011102230246251565404236316680908203125") == 0x3FF0000000000000u);
-        CHECK(native_bits64("1.00000000000000011102230246251565404236316680908203126") == 0x3FF0000000000001u);
-        CHECK(native_bits64("1.00000000000000011102230246251565404236316680908203124") == 0x3FF0000000000000u);
-        // subnormal and overflow boundaries
-        CHECK(native_bits64("2.4703282292062327e-324") == 0u);
-        CHECK(native_bits64("2.4703282292062328e-324") == 1u);
-        CHECK(native_bits64("2.2250738585072011e-308") == 0x000FFFFFFFFFFFFFu);
-        CHECK(native_bits64("2.2250738585072012e-308") == 0x0010000000000000u);
-        CHECK(native_bits64("1.7976931348623157e308") == 0x7FEFFFFFFFFFFFFFu);
-        CHECK(native_bits64("1.7976931348623159e308") == 0x7FF0000000000000u);
-        CHECK(native_bits64("-1e400") == 0xFFF0000000000000u);
-        CHECK(native_bits64("-1e-400") == 0x8000000000000000u);
-        // exponents and zeros far beyond the range cancel out
-        CHECK(native_bits64("0." + std::string(1000, '0') + "1e1001") == 0x3FF0000000000000u);
-        CHECK(native_bits64("1" + std::string(1000, '0') + "e-1000") == 0x3FF0000000000000u);
-        CHECK(native_bits64("1e-99999999999999999999999") == 0u);
-        CHECK(native_bits64("1E+99999999999999999999999") == 0x7FF0000000000000u);
-        // more digits than any midpoint has (769): only whether a nonzero digit follows matters
-        const std::string tie = "1.00000000000000011102230246251565404236316680908203125";
-        CHECK(native_bits64(tie + std::string(800, '0')) == 0x3FF0000000000000u);
-        CHECK(native_bits64(tie + std::string(800, '0') + "1") == 0x3FF0000000000001u);
-    }
-
-    SECTION("float")
-    {
-        CHECK(native_bits32("1.5") == 0x3FC00000u);
-        CHECK(native_bits32("0.1") == 0x3DCCCCCDu);
-        CHECK(native_bits32("-0.0") == 0x80000000u);
-        // 2^24 + 1 is exactly between two floats
-        CHECK(native_bits32("16777217") == 0x4B800000u);
-        CHECK(native_bits32("16777217.000000000000000000001") == 0x4B800001u);
-        CHECK(native_bits32("16777218.999999999999999999999") == 0x4B800001u);
-        CHECK(native_bits32("16777219") == 0x4B800002u);
-        // subnormal and overflow boundaries
-        CHECK(native_bits32("3.4028235677973366e38") == 0x7F7FFFFFu);
-        CHECK(native_bits32("3.4028235677973367e38") == 0x7F800000u);
-        CHECK(native_bits32("7.006492321624085e-46") == 0u);
-        CHECK(native_bits32("7.006492321624086e-46") == 1u);
-        CHECK(native_bits32("1.1754942e-38") == 0x007FFFFFu);
-        CHECK(native_bits32("-1.17549435e-38") == 0x80800000u);
-        CHECK(native_bits32("1e39") == 0x7F800000u);
-        CHECK(native_bits32("-1e-50") == 0x80000000u);
-        // not rounded through double: its double would round to another float
-        CHECK(native_bits32("1.00000005960464477539062500000000001") == 0x3F800001u);
-        CHECK(native_bits32("9007199254740993") == 0x5A000000u);
-    }
-
-    SECTION("the conversion shared with other parsers")
-    {
-        // convert_float() gives the lexer's results, for every type
-        const std::vector<std::string> tokens =
-        {
-            "0", "-0.0", "1.5", "0.1", "1e-400", "-2.5E+3", "123456789012345678901234567890",
-            "9007199254740993.0000000000000000001", "4.9406564584124654e-324"
-        };
-        using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
-        using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
-        for (const auto& t : tokens)
-        {
-            CAPTURE(t);
-            const auto layout = float_token_layout(t);
-            const char* const first = t.data();
-            const char* const last = first + t.size();
-            const auto d = nlohmann::detail::convert_float<double>(first, last, layout.first, layout.second);
-            const auto f = nlohmann::detail::convert_float<float>(first, last, layout.first, layout.second);
-            const auto ld = nlohmann::detail::convert_float<long double>(first, last, layout.first, layout.second);
-            CHECK(bits_of(d) == bits_of(json::parse(t).get<double>()));
-            CHECK(bits_of(f) == bits_of(float_json::parse(t).get<float>()));
-            CHECK(ld == long_double_json::parse(t).get<long double>());
-        }
-    }
+    // numbers that are not represented exactly on the fast path
+    CHECK_FALSE(fast("12345678901234567890", out));
+    CHECK_FALSE(fast("1e10000", out));
+    CHECK_FALSE(fast("9007199254740993", out));
+    CHECK_FALSE(fast("1e23", out));
+    CHECK_FALSE(fast("1e-23", out));
 }

 namespace
@@ -906,6 +806,40 @@ std::size_t big_bit_length(const big_uint& a)
    }
    return n;
 }
+
+std::uint64_t bits_of(double d)
+{
+    std::uint64_t b = 0;
+    std::memcpy(&b, &d, sizeof(b));
+    return b;
+}
+
+bool eisel_lemire(const std::string& s, double& out)
+{
+    return nlohmann::detail::parse_float_eisel_lemire(s.data(), s.data() + s.size(), out);
+}
+
+// significant digits of a token, without trailing zeros
+std::size_t significant_digits(const std::string& s)
+{
+    std::string digits;
+    for (const char c : s)
+    {
+        if (c == 'e' || c == 'E')
+        {
+            break;
+        }
+        if (c >= '0' && c <= '9' && !(digits.empty() && c == '0'))
+        {
+            digits += c;
+        }
+    }
+    while (!digits.empty() && digits.back() == '0')
+    {
+        digits.pop_back();
+    }
+    return digits.size();
+}
 } // namespace

 TEST_CASE("Eisel-Lemire float conversion")
@@ -1303,33 +1237,26 @@ TEST_CASE("Eisel-Lemire float conversion")
        for (const auto& c : known)
        {
            CAPTURE(c.first);
-            CHECK(native_bits64(c.first) == c.second);
+            double out = 0;
+            if (eisel_lemire(c.first, out))
+            {
+                CHECK(bits_of(out) == c.second);
+            }
+            else
+            {
+                // only tokens with more than 19 significant digits are left to
+                // strtod: those whose value lies too close to a tie
+                CHECK(significant_digits(c.first) > 19);
+            }
        }
    }

-    SECTION("binary32")
-    {
-        using binary32 = nlohmann::detail::ieee_binary_format<24>;
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(0, 1) == 0x3F800000u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-1, 1) == 0x3DCCCCCDu);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-1, 15) == 0x3FC00000u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(0, 16777217) == 0x4B800000u); // tie, to even
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(0, 16777219) == 0x4B800002u); // tie, to even
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-45, 1) == 0x00000001u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-46, 7) == 0x00000000u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-46, 8) == 0x00000001u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-65, 9999999999999999999u) == 0x00000000u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(20, 3402823466385288598u) == 0x7F7FFFFFu);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(20, 3402823669209384635u) == 0x7F800000u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(39, 1) == 0x7F800000u);
-        CHECK(nlohmann::detail::eisel_lemire<binary32>(-5, 0) == 0x00000000u);
-    }
-
    SECTION("round trip")
    {
-        // every double written by to_chars and read back, and its 17-digit
-        // form with trailing digits that make the token longer than 19 digits
+        // every double written by to_chars and read back, also with trailing
+        // digits that make the token longer than 19 digits
        std::uint64_t state = 5295;
+        std::size_t declined = 0;
        for (int i = 0; i < 200000; ++i)
        {
            state ^= state << 13u;
@@ -1351,51 +1278,30 @@ TEST_CASE("Eisel-Lemire float conversion")
            const char* end = nlohmann::detail::to_chars(buffer.data(), buffer.data() + buffer.size(), d);
            const std::string token(buffer.data(), static_cast<std::size_t>(end - buffer.data()));
            CAPTURE(token);
-            CHECK(native_bits64(token) == b);
+            double out = 0;
+            REQUIRE(eisel_lemire(token, out));
+            CHECK(bits_of(out) == b);

-            // insert digits before the exponent of the 17-digit form: that
-            // form lies strictly inside the rounding interval of the double
-            // (the shortest one may lie on its boundary), and the digits move
-            // it by far less than the distance to the boundary, so the value
-            // must not change
-            std::array<char, 64> digits17{};
-            static_cast<void>(std::snprintf(digits17.data(), digits17.size(), "%.17g", d)); // NOLINT(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
-            std::string longer = digits17.data();
+            // insert digits before the exponent: the value moves by far less
+            // than the distance to the rounding boundary, so it must not change
+            std::string longer = token;
            const std::size_t e = longer.find('e');
            const std::size_t dot = longer.find('.');
            const std::string extra = dot == std::string::npos ? ".000000000000000000001" : "000000000000000000001";
            longer.insert(e == std::string::npos ? longer.size() : e, extra);
            CAPTURE(longer);
-            CHECK(native_bits64(longer) == b);
-        }
-    }
-
-    SECTION("round trip, binary32")
-    {
-        std::uint32_t state = 5295;
-        for (int i = 0; i < 100000; ++i)
-        {
-            state ^= state << 13u;
-            state ^= state >> 17u;
-            state ^= state << 5u;
-            std::uint32_t b = state;
-            if ((b & 0x7F800000u) == 0x7F800000u)
+            if (eisel_lemire(longer, out))
            {
-                continue; // infinity or NaN
+                CHECK(bits_of(out) == b);
            }
-            if (i % 4 == 0)
+            else
            {
-                b &= 0x807FFFFFu; // subnormals
+                // w and w + 1 round differently: only when the value is very
+                // close to a rounding boundary
+                ++declined;
            }
-            float f = 0;
-            std::memcpy(&f, &b, sizeof(f));
-
-            std::array<char, 64> buffer{};
-            const char* end = nlohmann::detail::to_chars(buffer.data(), buffer.data() + buffer.size(), f);
-            const std::string token(buffer.data(), static_cast<std::size_t>(end - buffer.data()));
-            CAPTURE(token);
-            CHECK(native_bits32(token) == b);
        }
+        CHECK(declined < 1000); // 107 of the 200,000
    }

    SECTION("used by the lexer")
@@ -1409,177 +1315,3 @@ TEST_CASE("Eisel-Lemire float conversion")
                             "[json.exception.out_of_range.406] number overflow parsing '1.7976931348623159e308'", json::out_of_range&);
    }
 }
-
-namespace
-{
-using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
-
-// the bits of the float that parse() gives for a token, via both scanners;
-// the value must be the same for both
-template<typename Json, typename Bits>
-void check_parse(const std::string& token, Bits expected, Bits infinity)
-{
-    std::stringstream stream(token);
-    if ((expected & ~(Bits{1} << (8 * sizeof(Bits) - 1))) == infinity)
-    {
-        Json _;
-        CHECK_THROWS_WITH_AS(_ = Json::parse(token), ("[json.exception.out_of_range.406] number overflow parsing '" + token + "'").c_str(), typename Json::out_of_range&);
-        CHECK_THROWS_WITH_AS(_ = Json::parse(stream), ("[json.exception.out_of_range.406] number overflow parsing '" + token + "'").c_str(), typename Json::out_of_range&);
-        return;
-    }
-    const Json contiguous = Json::parse(token);
-    const Json streamed = Json::parse(stream);
-    if (contiguous.is_number_float()) // not an integer that fits
-    {
-        CHECK(bits_of(contiguous.template get<typename Json::number_float_t>()) == expected);
-        CHECK(bits_of(streamed.template get<typename Json::number_float_t>()) == expected);
-    }
-    else
-    {
-        CHECK(streamed.is_number_integer());
-    }
-}
-} // namespace
-
-TEST_CASE("float conversion of hard cases")
-{
-    // see float_hard_cases.hpp
-    for (const auto& c : float_hard_cases::cases())
-    {
-        const std::string token = c.token;
-        CAPTURE(token);
-        CHECK(native_bits64(token) == c.bits64);
-        CHECK(native_bits32(token) == c.bits32);
-        check_parse<json>(token, c.bits64, std::uint64_t{0x7FF0000000000000u});
-        check_parse<float_json>(token, c.bits32, std::uint32_t{0x7F800000u});
-    }
-}
-
-TEST_CASE("float overflow and underflow in the parser")
-{
-    SECTION("double")
-    {
-        check_parse<json>("1.7976931348623157e308", std::uint64_t{0x7FEFFFFFFFFFFFFFu}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("1.7976931348623159e308", std::uint64_t{0x7FF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("-1e309", std::uint64_t{0xFFF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("1" + std::string(400, '0'), std::uint64_t{0x7FF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("1e99999999999999999999", std::uint64_t{0x7FF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
-        // an underflow gives a zero with the sign of the token
-        check_parse<json>("1e-400", std::uint64_t{0}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("-1e-400", std::uint64_t{0x8000000000000000u}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("-2.4703282292062327e-324", std::uint64_t{0x8000000000000000u}, std::uint64_t{0x7FF0000000000000u});
-        check_parse<json>("0." + std::string(400, '0') + "1", std::uint64_t{0}, std::uint64_t{0x7FF0000000000000u});
-    }
-
-    SECTION("float")
-    {
-        check_parse<float_json>("3.4028234e38", std::uint32_t{0x7F7FFFFFu}, std::uint32_t{0x7F800000u});
-        check_parse<float_json>("3.4028236e38", std::uint32_t{0x7F800000u}, std::uint32_t{0x7F800000u});
-        check_parse<float_json>("-1e39", std::uint32_t{0xFF800000u}, std::uint32_t{0x7F800000u});
-        check_parse<float_json>("1e-46", std::uint32_t{0}, std::uint32_t{0x7F800000u});
-        check_parse<float_json>("-1e-46", std::uint32_t{0x80000000u}, std::uint32_t{0x7F800000u});
-        check_parse<float_json>("-7.006492321624085e-46", std::uint32_t{0x80000000u}, std::uint32_t{0x7F800000u});
-        check_parse<float_json>("-7.006492321624086e-46", std::uint32_t{0x80000001u}, std::uint32_t{0x7F800000u});
-    }
-}
-
-TEST_CASE("string scanning kernels")
-{
-    // the word-at-a-time kernels must stop exactly where a byte-by-byte scan
-    // stops, for any content, length, and alignment
-    const auto reference_special = [](const unsigned char* data, std::size_t n)
-    {
-        std::size_t i = 0;
-        while (i < n && !nlohmann::detail::is_string_special(data[i]))
-        {
-            ++i;
-        }
-        return i;
-    };
-    const auto reference_copyable = [](const unsigned char* data, std::size_t n)
-    {
-        std::size_t i = 0;
-        while (i < n && nlohmann::detail::is_ascii_copyable(data[i]))
-        {
-            ++i;
-        }
-        return i;
-    };
-    const auto reference_bulk_run = [](const unsigned char* data, std::size_t n)
-    {
-        std::size_t i = 0;
-        while (i < n)
-        {
-            if (data[i] < 0x80u)
-            {
-                if (nlohmann::detail::is_string_special(data[i]))
-                {
-                    break;
-                }
-                ++i;
-                continue;
-            }
-            const std::size_t seq = nlohmann::detail::validate_one_utf8(data + i, n - i);
-            if (seq == 0)
-            {
-                break;
-            }
-            i += seq;
-        }
-        return i;
-    };
-
-    // pieces: ordinary ASCII, stops, DEL, well-formed sequences of every
-    // length, and ill-formed or truncated ones
-    const std::vector<std::string> pieces =
-    {
-        "a", "Z", " ", "~", "0123456789", "\"", "\\", std::string(1, '\0'), "\n", "\x1F", "\x7F",
-        "\xC3\xA4", "\xE2\x82\xAC", "\xE6\x97\xA5\xE6\x9C\xAC", "\xF0\x9F\x98\x80", "\xED\x9F\xBF",
-        "\x80", "\xC0\x80", "\xC3", "\xE2\x82", "\xED\xA0\x80", "\xF4\x90\x80\x80", "\xFF",
-    };
-    std::uint64_t state = 5295;
-    const auto next = [&state]()
-    {
-        state ^= state << 13u;
-        state ^= state >> 7u;
-        state ^= state << 17u;
-        return state;
-    };
-    // the upper half as a 32-bit value: converts to std::size_t implicitly on
-    // every platform (a cast of std::uint64_t is useless where both are the
-    // same type, and required where std::size_t is 32 bits wide)
-    const auto next_small = [&next]()
-    {
-        return static_cast<std::uint32_t>(next() >> 32u);
-    };
-    for (int round = 0; round < 100000; ++round)
-    {
-        // mostly ordinary text, so that runs span several words
-        std::string text(next_small() % 8u, '.');
-        const std::size_t count = next_small() % 12u;
-        for (std::size_t k = 0; k < count; ++k)
-        {
-            const std::size_t p = (next() % 4 == 0) ? next_small() % pieces.size() : 0;
-            text += pieces[p];
-            text += std::string(next_small() % 10u, 'x');
-        }
-        const auto* data = reinterpret_cast<const unsigned char*>(text.data()); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
-        for (std::size_t offset = 0; offset < 3 && offset <= text.size(); ++offset)
-        {
-            const std::size_t n = text.size() - offset;
-            CAPTURE(text);
-            CAPTURE(offset);
-            CHECK(nlohmann::detail::find_string_special(data + offset, n) == reference_special(data + offset, n));
-            CHECK(nlohmann::detail::find_ascii_copyable_run(data + offset, n) == reference_copyable(data + offset, n));
-            CHECK(nlohmann::detail::scalar_string_bulk_run(data + offset, n) == reference_bulk_run(data + offset, n));
-        }
-    }
-
-    // the trailing-zero count, whichever implementation the compiler gets
-    for (int k = 0; k < 64; ++k)
-    {
-        const std::uint64_t bit = std::uint64_t{1} << k;
-        CHECK(nlohmann::detail::count_trailing_zeros(bit) == k);
-        CHECK(nlohmann::detail::count_trailing_zeros(bit | (bit << 1u) | 0x8000000000000000u) == k);
-    }
-}
@@ -1973,4 +1973,115 @@ TEST_CASE("operator[] with user-defined std::string_view-convertible types")
        }
    }
 }
+
+TEST_CASE("keys convertible to std::string_view work with all lookup functions (regression test for #5663)")
+{
+    // a key type convertible only to std::string_view: the case #4958 added
+    // support for, but only the non-const operator[] compiled with it
+    struct ViewKey
+    {
+        operator std::string_view() const
+        {
+            return "a";
+        }
+    };
+
+    // a key type convertible to both std::string and std::string_view: with
+    // 3.12.0, such a key worked with at, the const operator[], find, count and
+    // contains via the conversion to std::string; #4958 made the KeyType&&
+    // templates win overload resolution for it instead, and those then failed
+    struct DualKey
+    {
+        operator std::string() const
+        {
+            return "a";
+        }
+        operator std::string_view() const
+        {
+            return "a";
+        }
+    };
+
+    SECTION("nlohmann::json")
+    {
+        using json = nlohmann::json;
+
+        SECTION("ViewKey")
+        {
+            json j = {{"a", 1}};
+            const json& cj = j;
+
+            CHECK(j[ViewKey{}] == 1);
+            CHECK(cj[ViewKey{}] == 1);
+            CHECK(j.at(ViewKey{}) == 1);
+            CHECK(cj.at(ViewKey{}) == 1);
+            CHECK(j.find(ViewKey{}) != j.end());
+            CHECK(cj.find(ViewKey{}) != cj.end());
+            CHECK(j.count(ViewKey{}) == 1);
+            CHECK(j.contains(ViewKey{}));
+            CHECK(j.value(ViewKey{}, 0) == 1);
+            CHECK(j.erase(ViewKey{}) == 1);
+            CHECK(!j.contains("a"));
+        }
+
+        SECTION("DualKey")
+        {
+            json j = {{"a", 1}};
+            const json& cj = j;
+
+            CHECK(j[DualKey{}] == 1);
+            CHECK(cj[DualKey{}] == 1);
+            CHECK(j.at(DualKey{}) == 1);
+            CHECK(cj.at(DualKey{}) == 1);
+            CHECK(j.find(DualKey{}) != j.end());
+            CHECK(cj.find(DualKey{}) != cj.end());
+            CHECK(j.count(DualKey{}) == 1);
+            CHECK(j.contains(DualKey{}));
+            CHECK(j.value(DualKey{}, 0) == 1);
+            CHECK(j.erase(DualKey{}) == 1);
+            CHECK(!j.contains("a"));
+        }
+    }
+
+    SECTION("nlohmann::ordered_json")
+    {
+        using ordered_json = nlohmann::ordered_json;
+
+        SECTION("ViewKey")
+        {
+            ordered_json j = {{"a", 1}};
+            const ordered_json& cj = j;
+
+            CHECK(j[ViewKey{}] == 1);
+            CHECK(cj[ViewKey{}] == 1);
+            CHECK(j.at(ViewKey{}) == 1);
+            CHECK(cj.at(ViewKey{}) == 1);
+            CHECK(j.find(ViewKey{}) != j.end());
+            CHECK(cj.find(ViewKey{}) != cj.end());
+            CHECK(j.count(ViewKey{}) == 1);
+            CHECK(j.contains(ViewKey{}));
+            CHECK(j.value(ViewKey{}, 0) == 1);
+            CHECK(j.erase(ViewKey{}) == 1);
+            CHECK(!j.contains("a"));
+        }
+
+        SECTION("DualKey")
+        {
+            ordered_json j = {{"a", 1}};
+            const ordered_json& cj = j;
+
+            CHECK(j[DualKey{}] == 1);
+            CHECK(cj[DualKey{}] == 1);
+            CHECK(j.at(DualKey{}) == 1);
+            CHECK(cj.at(DualKey{}) == 1);
+            CHECK(j.find(DualKey{}) != j.end());
+            CHECK(cj.find(DualKey{}) != cj.end());
+            CHECK(j.count(DualKey{}) == 1);
+            CHECK(j.contains(DualKey{}));
+            CHECK(j.value(DualKey{}, 0) == 1);
+            CHECK(j.erase(DualKey{}) == 1);
+            CHECK(!j.contains("a"));
+        }
+    }
+}
 #endif
@@ -257,11 +257,10 @@ struct LocaleSwitchingSax final: public nlohmann::json_sax<json>

 TEST_CASE("locale changes between lexer construction and number conversion (#5198)")
 {
-    // float and double are converted without the locale. A long double that
-    // is not binary64 can take the strtold fallback, which honors the locale
-    // that is current at conversion time. The numbers are chosen so that it
-    // does: too many significant digits for Clinger's fast path, an underflow
-    // that std::from_chars rejects, and a plain value.
+    // The numbers are chosen so that the conversion also takes the strtod
+    // fallback, which honors the locale that is current at conversion time:
+    // too many significant digits for Clinger's fast path, an underflow that
+    // std::from_chars rejects, and a plain value.
    const std::vector<std::string> numbers = {"3.14159265358979323846", "1.5e-400", "12.34", "-0.000123456789012345678"};
    std::string text = "[";
    for (const auto& n : numbers)
@@ -325,8 +324,7 @@ TEST_CASE("locale changes between lexer construction and number conversion (#519
            }
        }

-        // a long double goes through std::strtold unless it is binary64 or
-        // std::from_chars supports it
+        // a long double goes through std::strtold unless std::from_chars supports it
        {
            bool switched = false;
            const auto cb = [&](int /*depth*/, long_double_json::parse_event_t event, long_double_json& /*parsed*/) noexcept
@@ -352,15 +350,8 @@ TEST_CASE("locale with a multi-byte decimal point")
 {
    // Some locales use a decimal point that is not a single character, e.g.
    // U+066B ARABIC DECIMAL SEPARATOR (two bytes in UTF-8). It cannot be
-    // substituted in place for '.', so the strtold fallback (only for long
-    // double formats other than binary64) converts a copy of the token with
-    // the whole decimal point instead (#5660). The values must be those of the
-    // "C" locale.
-    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
-    const char* const long_double_numbers = "[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]";
-    REQUIRE(std::setlocale(LC_NUMERIC, "C") != nullptr);
-    const long_double_json expected_long_double = long_double_json::parse(long_double_numbers);
-
+    // substituted in place for '.', so the strtod fallback stops early. The
+    // conversion must still terminate rather than retry forever.
    const std::array<const char*, 6> names = {{"ar_EG.UTF-8", "ar_SA.UTF-8", "fa_IR.UTF-8", "ps_AF.UTF-8", "ar_EG", "fa_IR"}};
    bool tested = false;
    for (const char* name : names)
@@ -378,20 +369,12 @@ TEST_CASE("locale with a multi-byte decimal point")
        tested = true;

        // too many significant digits for Clinger's fast path, and an underflow
-        // that std::from_chars rejects: double does not depend on the locale
+        // that std::from_chars rejects: both reach the strtod fallback
        json j;
        CHECK_NOTHROW(j = json::parse("[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]"));
        CHECK(j.is_array());
-        CHECK(j[0] == 3.14159265358979323846);
-        CHECK(j[1] == 0.0);
-        CHECK(j[2] == -0.000123456789012345678);
        CHECK(json::accept("3.14159265358979323846"));

-        // a long double that reaches the strtold fallback is not truncated
-        long_double_json ld;
-        CHECK_NOTHROW(ld = long_double_json::parse(long_double_numbers));
-        CHECK(ld == expected_long_double);
-
        // a value the locale-independent paths convert is not affected
        CHECK(json::parse("12.5") == 12.5);
    }