Fix CI failures in from_chars locale handling (PR #5237)

Three issues in the extended-locale number parsing were breaking CI:

- On Windows, _create_locale() was called with LC_ALL_MASK, a POSIX-only
  constant that doesn't exist in the Windows CRT; it needs LC_ALL there.
- `::isspace_l`/`::_isspace_l` were called with a leading `::`, but on
  glibc, mingw-w64 and some other C libraries these are macros that expand
  to a parenthesized expression, so `::` followed by the expansion is a
  syntax error ("expected unqualified-id"). Dropping the `::` fixes this
  on all affected toolchains (gcc/clang on Linux, nvcc, mingw).
- The Windows implementation assumed the full `_<funcname>_l` family is
  always available, but some MinGW toolchains only provide a subset
  (e.g. missing `_strtoll_l`/`_strtoull_l`/`_strtof_l`/`_strtold_l`).
  The extended-locale fast path is now only used for genuine MSVC
  (including clang-cl, which mimics the MSVC ABI/CRT); other Windows
  toolchains fall back to the locale-dependent standard functions.

Also fixes a clang-tidy hicpp-named-parameter/readability-named-parameter
warning-as-error on two unnamed `init_val_t` test parameters.

Verified locally with gcc/clang on Linux, clang++ in C++20 mode, and
mingw-w64 (both g++ and clang targeting x86_64-w64-mingw32).

BUILD.bazel

@@ -21,6 +21,7 @@ cc_library(
         "include/nlohmann/adl_serializer.hpp",
         "include/nlohmann/byte_container_with_subtype.hpp",
         "include/nlohmann/detail/abi_macros.hpp",
+        "include/nlohmann/detail/conversions/from_chars.hpp",
         "include/nlohmann/detail/conversions/from_json.hpp",
         "include/nlohmann/detail/conversions/to_chars.hpp",
         "include/nlohmann/detail/conversions/to_json.hpp",

include/nlohmann/detail/conversions/from_chars.hpp

@@ -0,0 +1,636 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  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 <nlohmann/detail/macro_scope.hpp>
+
+#if JSON_HAS_FROM_CHARS
+    #include <algorithm> // find, find_if
+    #include <charconv> // from_chars, from_chars_result
+    #include <cmath> // isnan
+#else
+    #include <cctype> // isspace
+    #include <cerrno> // ERANGE
+    #include <clocale> // localeconv, newlocale, freelocale
+    #include <cstdlib> // strto*
+    #include <memory> // unique_ptr
+    #include <type_traits> // enable_if, is_unsigned, remove_pointer
+    #include <utility> // declval
+
+    #ifdef __has_include
+        #if __has_include(<xlocale.h>)
+            #include <xlocale.h> // strto*_l, isspace_l
+        #endif
+    #endif
+#endif
+#include <limits> // numeric_limits<T>::[has_]infinity, quiet_NaN
+#include <system_error> // errc
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+
+constexpr char get_locale_independent_decimal_point() noexcept
+{
+    return '.';
+}
+
+#if JSON_HAS_FROM_CHARS
+
+//////////////////////////////////////////////
+// Delegate to std::from_chars if supported //
+//////////////////////////////////////////////
+
+/*!
+@brief Number parsing implementation details
+
+A traits class template that allows users of nlohmann::detail::from_chars to
+query details of the used implementation.
+
+@tparam T numeric type to parse, matching the `value` argument of from_chars
+*/
+template <typename T>
+struct from_chars_traits
+{
+    /// whether the from_chars in unaffected by the current global C locale
+    static constexpr bool is_locale_independent = true;
+
+    /// getter for the character used as decimal point by from_chars
+    static constexpr char(*get_decimal_point)() = &get_locale_independent_decimal_point;
+};
+
+using std::from_chars_result;
+
+/*!
+@brief Parse integer/floating-point numbers
+
+Parses the string [first, last) into the numeric type T.
+
+This implementation merely delegates to `std::from_chars` with one noteworthy
+difference: Different C++ standard library implementations do not agree on
+whether `value` should be set in the out-of-range case for floating-point
+numbers. Presently, libstdc++ leaves `value` unchanged whereas libc++ and MS STL
+set `value` to +/-0 or +/-inf which allows its users to distinguish between the
+various cases of over- and underflow. The C++ proposal [P4168][1] details this
+discrepancy and advocates to standardize the latter behavior.
+
+Since we need to be able to distinguish absolute underflow (+/-0) from absolute
+overflow (+/-inf), this implementation "corrects" the out-of-range behavior
+accordingly by estimating the effective exponent through manual string parsing.
+
+[1]: https://isocpp.org/files/papers/P4168R0.html
+
+@tparam T numeric type to parse
+@param[in] first points to the beginning of the parsed string (inclusive)
+@param[in] end points to the end of the parsed string (exclusive)
+@param[out] value references the variable to set the parsed number
+@return structure consisting of `ptr` and `ec` such that:
+        - If the string starting at `first` matches a number, `ptr` points to
+          the address immediately after the last character that matches.
+          * If the matched number can be represented by `T`, `value` is set and
+            `ec` is value-initialized.
+          * If the matched number cannot be represented by `T`, `ec` is set to
+            `std::errc::result_out_of_range` and depending on the type of `T`:
+            + `value` is left unchanged for integral `T`,
+            + `value` is set to +/-0 or +/-inf for floating-point `T`.
+        - If the string starting at `first` doesn't match a number, `ptr`
+          points to `first`, `ec` is `std::errc::invalid_argument`, and `value`
+          is left unchanged.
+*/
+template <typename T>
+JSON_HEDLEY_NON_NULL(1, 2)
+inline from_chars_result from_chars(const char* first, const char* last, T& value)
+{
+    // No implementation of std::from_chars will set its value argument to NaN,
+    // so by using NaN as the initial value, we can tell if it changed.
+    T inner_value = std::numeric_limits<T>::quiet_NaN();
+    auto res = std::from_chars(first, last, inner_value);
+
+    if constexpr (std::numeric_limits<T>::has_infinity)
+    {
+        if (res.ec == std::errc::result_out_of_range)
+        {
+            if (std::isnan(inner_value)) // inner_value was not set
+            {
+                // [first, res.ptr) matches a valid floating-point number that's
+                // out-of-range and our std::from_chars did not set `value`, so we
+                // need to distinguish the type of under-/overflow ourselves.
+                const char* mantissa_begin = *first == '-' ? first + 1 : first;
+                const char* mantissa_end = std::find_if(mantissa_begin, res.ptr, [](char c)
+                {
+                    return c == 'e' || c == 'E';
+                });
+                const char* decimal_point = std::find(mantissa_begin, mantissa_end, '.');
+                const char* significant_digit = std::find_if(mantissa_begin, mantissa_end, [](char d)
+                {
+                    return d >= '1' && d <= '9';
+                });
+
+                // Position of the significant digit relative to the decimal gives
+                // the order of magnitude of the mantissa.
+                auto effective_exponent = static_cast<int>(decimal_point - significant_digit);
+
+                // If the number includes an explicit exponent, add that to the the
+                // total exponent.
+                if (mantissa_end != res.ptr)
+                {
+                    const char* exponent_begin = *(mantissa_end + 1) == '+'
+                                                 ? mantissa_end + 2 // skip the exponent's + sign
+                                                 : mantissa_end + 1;
+                    int exponent = 0;
+                    auto exp_res = std::from_chars(exponent_begin, res.ptr, exponent);
+                    JSON_ASSERT(exp_res.ptr == res.ptr);
+
+                    if (exp_res.ec == std::errc::result_out_of_range)
+                    {
+                        // NOTE: Parentheses around function names mitigate min/max macro collision on Windows
+                        effective_exponent = *exponent_begin == '-'
+                                             ? (std::numeric_limits<int>::min)()
+                                             : (std::numeric_limits<int>::max)();
+                    }
+                    else
+                    {
+                        JSON_ASSERT(exp_res.ec == std::errc{});
+                        effective_exponent += exponent;
+                    }
+                }
+
+                // Set `value` to the sign-correct non-finite value based on the
+                // effective exponent.
+                value = (*first == '-' ? -1 : 1) * (effective_exponent < 0
+                                                    ? T{}
+                                                    : std::numeric_limits<T>::infinity());
+            }
+            else
+            {
+                value = inner_value;
+            }
+        }
+    }
+
+    if (res.ec == std::errc{})
+    {
+        value = inner_value;
+    }
+
+    return res;
+}
+
+#else
+
+//////////////////////////////////////////////
+// Fallback implementation using strto*[_l] //
+//////////////////////////////////////////////
+
+#if defined(_WIN32)
+
+/*!
+@brief Extended locale functions on Windows
+
+A trait object which provides wrappers for isspace and the strto* family of
+functions, based on the platform's support for extended locale APIs.
+
+This is the Windows implementation. The primary class template definition
+falls back to the standard locale-dependent functions, which is used on
+MinGW, whose C runtime only provides an incomplete subset of the
+`_<funcname>_l` family (e.g., missing `_strtof_l`/`_strtold_l` depending on
+the toolchain version). A specialization for genuine MSVC (including
+clang-cl, which mimics the MSVC ABI and CRT) is used instead, as the full
+`_<funcname>_l` family has reliably been available there since at least
+Visual Studio 2005.
+*/
+template <typename = const char*, typename = float>
+struct extended_locale_traits
+{
+    static constexpr bool is_locale_independent = false;
+
+    JSON_HEDLEY_PURE
+    static char get_decimal_point() noexcept
+    {
+        const auto* loc = localeconv();
+        JSON_ASSERT(loc != nullptr);
+        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
+    }
+
+    static int isspace(int c) noexcept
+    {
+        return std::isspace(c);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return std::strtoll(str, endptr, 10);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return std::strtoull(str, endptr, 10);
+    }
+
+    static constexpr float(&strtof)(const char*, char**) = std::strtof;
+    static constexpr double(&strtod)(const char*, char**) = std::strtod;
+    static constexpr long double(&strtold)(const char*, char**) = std::strtold;
+};
+
+#if defined(_MSC_VER)
+
+/*!
+@brief Get a pointer to a globally shared instance of the "C" locale on Windows
+
+This function uses _create_locale/_free_locale to allocate a "C" locale instance
+which can be passed to extended locale APIs. This instance is created on first
+use and has static storage duration, such that it can be used for the duration
+of the program.
+*/
+inline _locale_t get_c_locale_t() noexcept
+{
+    struct LocaleTDeleter
+    {
+        void operator()(_locale_t loc) noexcept
+        {
+            ::_free_locale(loc);
+        }
+    };
+    using LocaleUPtr = std::unique_ptr<std::remove_pointer<_locale_t>::type, LocaleTDeleter>;
+    static const LocaleUPtr c_locale = LocaleUPtr{::_create_locale(LC_ALL, "C"), LocaleTDeleter{}};
+    return c_locale.get();
+}
+
+template <typename T>
+struct extended_locale_traits<T, float>
+{
+    static constexpr bool is_locale_independent = true;
+    static constexpr char(&get_decimal_point)() = get_locale_independent_decimal_point;
+
+    static int isspace(int c) noexcept
+    {
+        return _isspace_l(c, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return _strtoll_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return _strtoull_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    static float strtof(T str, char** str_end)
+    {
+        return _strtof_l(str, str_end, get_c_locale_t());
+    }
+
+    static double strtod(T str, char** str_end)
+    {
+        return _strtod_l(str, str_end, get_c_locale_t());
+    }
+
+    static long double strtold(T str, char** str_end)
+    {
+        return _strtold_l(str, str_end, get_c_locale_t());
+    }
+};
+
+#endif
+
+#else
+
+/*!
+@brief Get a pointer to a globally shared instance of the "C" locale on POSIX
+
+This function uses newlocale/freelocale to allocate a "C" locale instance
+which can be passed to extended locale APIs. This instance is created on first
+use and has static storage duration, such that it can be used for the duration
+of the program.
+
+newlocale/freelocale themselves are in POSIX and are available independent of
+the platform's support for extended locale APIs.
+*/
+inline locale_t get_c_locale_t() noexcept
+{
+    struct LocaleTDeleter
+    {
+        void operator()(locale_t loc) noexcept
+        {
+            ::freelocale(loc);
+        }
+    };
+    using LocaleUPtr = std::unique_ptr<std::remove_pointer<locale_t>::type, LocaleTDeleter>;
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wexit-time-destructors"
+#endif
+    static const LocaleUPtr c_locale = LocaleUPtr {::newlocale(LC_ALL_MASK, "C", nullptr), LocaleTDeleter{}};
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
+    return c_locale.get();
+}
+
+/*!
+@brief Extended locale functions on POSIX
+
+A trait object which provides wrappers for isspace and the strto* family of
+functions, based on the platform's support for extended locale APIs.
+
+This is the POSIX implementation where extended locale support might not be
+available. The primary class template definition falls back to the standard
+locale-dependent functions. A partial specialization uses SFINAE to detect the
+availability of `strtof_l` (as a proxy for the whole `<funcname>_l` family) and
+provides access to locale-independent functions.
+*/
+template <typename = const char*, typename = float>
+struct extended_locale_traits
+{
+    static constexpr bool is_locale_independent = false;
+
+    /*!
+    @brief Query the decimal point used by the current C locale
+
+    Note that calling this function while switching the global C locale from
+    another thread is undefined behavior.
+    */
+    JSON_HEDLEY_PURE
+    static char get_decimal_point() noexcept
+    {
+        const auto* loc = localeconv();
+        JSON_ASSERT(loc != nullptr);
+        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
+    }
+
+    static int isspace(int c) noexcept
+    {
+        return std::isspace(c);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return std::strtoll(str, endptr, 10);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return std::strtoull(str, endptr, 10);
+    }
+
+    static constexpr float(&strtof)(const char*, char**) = std::strtof;
+    static constexpr double(&strtod)(const char*, char**) = std::strtod;
+    static constexpr long double(&strtold)(const char*, char**) = std::strtold;
+};
+
+template <typename T>
+struct extended_locale_traits<T, decltype(strtof_l(
+            std::declval<T>(),
+            std::declval<char**>(),
+            std::declval<locale_t>()))>
+{
+    static constexpr bool is_locale_independent = true;
+    static constexpr char(&get_decimal_point)() = get_locale_independent_decimal_point;
+
+    static int isspace(int c) noexcept
+    {
+        return isspace_l(c, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return ::strtoll_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return ::strtoull_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    static float strtof(T str, char** str_end)
+    {
+        return ::strtof_l(str, str_end, get_c_locale_t());
+    }
+
+    static double strtod(T str, char** str_end)
+    {
+        return ::strtod_l(str, str_end, get_c_locale_t());
+    }
+
+    static long double strtold(T str, char** str_end)
+    {
+        return ::strtold_l(str, str_end, get_c_locale_t());
+    }
+};
+
+#endif
+
+/*!
+@brief Number parsing implementation details
+
+A traits class template that allows users of nlohmann::detail::from_chars to
+query details of the used implementation.
+
+Each specialization provides the following static members:
+- is_locale_independent: whether the from_chars in unaffected by the current
+                         global C locale
+- get_decimal_point: getter for the character used as decimal point by from_chars
+- strto: dispatches to the C standard library strto* function for type T, or to
+         the corresponding extended locale function, if available.
+
+@tparam T numeric type to parse, matching the `value` argument of from_chars
+*/
+template <typename T>
+struct from_chars_traits;
+
+template <>
+struct from_chars_traits<long long> // NOLINT(runtime/int)
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static constexpr long long(*strto)(const char*, char**) = &extended_locale_traits<>::strtoll;
+};
+
+template <>
+struct from_chars_traits<unsigned long long> // NOLINT(runtime/int)
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static constexpr unsigned long long(*strto)(const char*, char**) = &extended_locale_traits<>::strtoull;
+};
+
+template <>
+struct from_chars_traits<float>
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    static constexpr float(*strto)(const char*, char**) = &extended_locale_traits<>::strtof;
+};
+
+template <>
+struct from_chars_traits<double>
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    static constexpr double(*strto)(const char*, char**) = &extended_locale_traits<>::strtod;
+};
+
+template <>
+struct from_chars_traits<long double>
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    static constexpr long double(*strto)(const char*, char**) = &extended_locale_traits<>::strtold;
+};
+
+template <typename T>
+constexpr typename std::enable_if<std::numeric_limits<T>::has_infinity, bool>::type is_out_of_range_value(T value) noexcept
+{
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wfloat-equal"
+#endif
+    return value == T {} || value == std::numeric_limits<T>::infinity() || value == -std::numeric_limits<T>::infinity();
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
+}
+
+template <typename T>
+constexpr typename std::enable_if < !std::numeric_limits<T>::has_infinity, bool >::type is_out_of_range_value(T value) noexcept
+{
+    // NOTE: Parentheses around function names mitigate min/max macro collision on Windows
+    return value == (std::numeric_limits<T>::max)() || value == (std::numeric_limits<T>::min)();
+}
+
+struct from_chars_result
+{
+    const char* ptr;
+    std::errc ec;
+};
+
+/*!
+@brief Parse integer/floating-point numbers
+
+Parses the string [first, last) into the numeric type T.
+
+This implementation uses the C standard library functions strto*, or their
+extended locale counterparts strto*_l, to emulate the behavior of
+`std::from_chars` on platforms where it is not (fully) supported.
+
+Regarding the under-/overflow behavior, this implementation adopts the behavior
+proposed by [P4168][1] and implemented by libc++ and MS STL of setting `value`
+to the corresponding non-finite floating-point value in case of an out-of-range
+result.
+
+[1]: https://isocpp.org/files/papers/P4168R0.html
+
+@pre Unlike std::from_chars, this implementation requires the string to be
+     null-terminated, such that `last` dereferences into a NUL byte.
+
+@tparam T numeric type to parse
+@param[in] first points to the beginning of the parsed string (inclusive)
+@param[in] end points to the end of the parsed string (exclusive)
+@param[out] value references the variable to set the parsed number
+@return structure consisting of `ptr` and `ec` such that:
+        - If the string starting at `first` matches a number, `ptr` points to
+          the address immediately after the last character that matches.
+          * If the matched number can be represented by `T`, `value` is set and
+            `ec` is value-initialized.
+          * If the matched number cannot be represented by `T`, `ec` is set to
+            `std::errc::result_out_of_range` and depending on the type of `T`:
+            + `value` is left unchanged for integral `T`,
+            + `value` is set to +/-0 or +/-inf for floating-point `T`.
+        - If the string starting at `first` doesn't match a number, `ptr`
+          points to `first`, `ec` is `std::errc::invalid_argument`, and `value`
+          is left unchanged.
+*/
+template <typename T>
+JSON_HEDLEY_NON_NULL(1, 2)
+inline from_chars_result from_chars(const char* first, const char* last, T& value)
+{
+    JSON_ASSERT(*last == '\0');
+
+    // Unlike strto*, from_chars does not accept leading whitespace or + signs
+    if (first == last || *first == '+'
+            || (std::is_unsigned<T>::value && *first == '-')
+            || extended_locale_traits<>::isspace(*first) != 0)
+    {
+        return {first, std::errc::invalid_argument};
+    }
+
+    errno = 0;
+    char* ptr = nullptr; // NOLINT(misc-const-correctness)
+    T result = from_chars_traits<T>::strto(first, &ptr);
+
+    if (ptr == first)
+    {
+        return {ptr, std::errc::invalid_argument};
+    }
+
+    // Upon under-/overflow, strto* returns a marginal value and sets errno.
+    // Note that it is NOT sufficient to just check errno: strto* only clears
+    // errno if the parsed string actually parses into 0/[U]LLONG_MIN/-_MAX
+    // and this result was returned without indicating an under-/overflow.
+    // Otherwise, errno may not be relied upon to indicate the *absence* of
+    // an out-of-range error.
+    if (is_out_of_range_value(result) && errno == ERANGE)
+    {
+        if (std::numeric_limits<T>::has_infinity)
+        {
+            // ONLY for floating-point types, set `value` to the same non-finite
+            // result returned by strto* to allow users to distinguish between
+            // different types of under-/overflow.
+            value = result;
+        }
+        return {ptr, std::errc::result_out_of_range};
+    }
+
+    value = result;
+    return {ptr, std::errc{}};
+}
+
+#endif
+
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/input/lexer.hpp

@@ -9,15 +9,15 @@
 #pragma once
 
 #include <array> // array
-#include <clocale> // localeconv
 #include <cstddef> // size_t
 #include <cstdio> // snprintf
-#include <cstdlib> // strtof, strtod, strtold, strtoll, strtoull
 #include <initializer_list> // initializer_list
 #include <string> // char_traits, string
+#include <system_error> // errc
 #include <utility> // move
 #include <vector> // vector
 
+#include <nlohmann/detail/conversions/from_chars.hpp>
 #include <nlohmann/detail/input/input_adapters.hpp>
 #include <nlohmann/detail/input/position_t.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
@@ -152,7 +152,7 @@ class lexer : public lexer_base<BasicJsonType>
     explicit lexer(InputAdapterType&& adapter, bool ignore_comments_ = false) noexcept
         : ia(std::move(adapter))
         , ignore_comments(ignore_comments_)
-        , decimal_point_char(static_cast<char_int_type>(get_decimal_point()))
+        , decimal_point_char(static_cast<char_int_type>(from_chars_traits<number_float_t>::get_decimal_point()))
     {}
 
     // deleted because of pointer members
@@ -163,19 +163,6 @@ class lexer : public lexer_base<BasicJsonType>
     ~lexer() = default;
 
   private:
-    /////////////////////
-    // locales
-    /////////////////////
-
-    /// return the locale-dependent decimal point
-    JSON_HEDLEY_PURE
-    static char get_decimal_point() noexcept
-    {
-        const auto* loc = localeconv();
-        JSON_ASSERT(loc != nullptr);
-        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
-    }
-
     /////////////////////
     // scan functions
     /////////////////////
@@ -941,24 +928,6 @@ class lexer : public lexer_base<BasicJsonType>
         }
     }
 
-    JSON_HEDLEY_NON_NULL(2)
-    static void strtof(float& f, const char* str, char** endptr) noexcept
-    {
-        f = std::strtof(str, endptr);
-    }
-
-    JSON_HEDLEY_NON_NULL(2)
-    static void strtof(double& f, const char* str, char** endptr) noexcept
-    {
-        f = std::strtod(str, endptr);
-    }
-
-    JSON_HEDLEY_NON_NULL(2)
-    static void strtof(long double& f, const char* str, char** endptr) noexcept
-    {
-        f = std::strtold(str, endptr);
-    }
-
     /*!
     @brief scan a number literal
 
@@ -1279,19 +1248,18 @@ scan_number_done:
         // we are done scanning a number)
         unget();
 
-        char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
-        errno = 0;
-
         // try to parse integers first and fall back to floats
         if (number_type == token_type::value_unsigned)
         {
-            const auto x = std::strtoull(token_buffer.data(), &endptr, 10);
+            unsigned long long x{}; // NOLINT(runtime/int)
+            const auto res = ::nlohmann::detail::from_chars(token_buffer.data(), token_buffer.data() + token_buffer.size(), x);
 
             // we checked the number format before
-            JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
+            JSON_ASSERT(res.ptr == token_buffer.data() + token_buffer.size());
 
-            if (errno != ERANGE)
+            if (res.ec != std::errc::result_out_of_range)
             {
+                JSON_ASSERT(res.ec == std::errc{});
                 value_unsigned = static_cast<number_unsigned_t>(x);
                 if (value_unsigned == x)
                 {
@@ -1301,13 +1269,15 @@ scan_number_done:
         }
         else if (number_type == token_type::value_integer)
         {
-            const auto x = std::strtoll(token_buffer.data(), &endptr, 10);
+            long long x{}; // NOLINT(runtime/int)
+            const auto res = ::nlohmann::detail::from_chars(token_buffer.data(), token_buffer.data() + token_buffer.size(), x);
 
             // we checked the number format before
-            JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
+            JSON_ASSERT(res.ptr == token_buffer.data() + token_buffer.size());
 
-            if (errno != ERANGE)
+            if (res.ec != std::errc::result_out_of_range)
             {
+                JSON_ASSERT(res.ec == std::errc{});
                 value_integer = static_cast<number_integer_t>(x);
                 if (value_integer == x)
                 {
@@ -1318,10 +1288,11 @@ scan_number_done:
 
         // this code is reached if we parse a floating-point number or if an
         // integer conversion above failed
-        strtof(value_float, token_buffer.data(), &endptr);
+        const auto res = ::nlohmann::detail::from_chars(token_buffer.data(), token_buffer.data() + token_buffer.size(), value_float);
 
         // we checked the number format before
-        JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
+        JSON_ASSERT(res.ptr == token_buffer.data() + token_buffer.size());
+        JSON_ASSERT(res.ec != std::errc::invalid_argument);
 
         return token_type::value_float;
     }

include/nlohmann/detail/macro_scope.hpp

@@ -124,6 +124,20 @@
     #define JSON_HAS_FILESYSTEM 0
 #endif
 
+#ifndef JSON_HAS_FROM_CHARS
+    #if defined(JSON_HAS_CPP_17) && defined(__cpp_lib_to_chars)
+        // The std::from_chars<float> implementation in libstdc++ 11 gets some of the corner cases wrong;
+        // starting with libstdc++ 12, these are fixed by switching the implementation to fast_float.
+        #if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 12
+            #define JSON_HAS_FROM_CHARS 0
+        #else
+            #define JSON_HAS_FROM_CHARS 1
+        #endif
+    #else
+        #define JSON_HAS_FROM_CHARS 0
+    #endif
+#endif
+
 #ifndef JSON_HAS_THREE_WAY_COMPARISON
     #if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201907L \
         && defined(__cpp_lib_three_way_comparison) && __cpp_lib_three_way_comparison >= 201907L

include/nlohmann/detail/macro_unscope.hpp

@@ -39,6 +39,7 @@
     #undef JSON_HAS_CPP_26
     #undef JSON_HAS_FILESYSTEM
     #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
+    #undef JSON_HAS_FROM_CHARS
     #undef JSON_HAS_THREE_WAY_COMPARISON
     #undef JSON_HAS_RANGES
     #undef JSON_HAS_STD_FORMAT

single_include/nlohmann/json.hpp

@@ -2499,6 +2499,20 @@ JSON_HEDLEY_DIAGNOSTIC_POP
     #define JSON_HAS_FILESYSTEM 0
 #endif
 
+#ifndef JSON_HAS_FROM_CHARS
+    #if defined(JSON_HAS_CPP_17) && defined(__cpp_lib_to_chars)
+        // The std::from_chars<float> implementation in libstdc++ 11 gets some of the corner cases wrong;
+        // starting with libstdc++ 12, these are fixed by switching the implementation to fast_float.
+        #if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 12
+            #define JSON_HAS_FROM_CHARS 0
+        #else
+            #define JSON_HAS_FROM_CHARS 1
+        #endif
+    #else
+        #define JSON_HAS_FROM_CHARS 0
+    #endif
+#endif
+
 #ifndef JSON_HAS_THREE_WAY_COMPARISON
     #if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201907L \
         && defined(__cpp_lib_three_way_comparison) && __cpp_lib_three_way_comparison >= 201907L
@@ -7690,15 +7704,653 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 #include <array> // array
-#include <clocale> // localeconv
 #include <cstddef> // size_t
 #include <cstdio> // snprintf
-#include <cstdlib> // strtof, strtod, strtold, strtoll, strtoull
 #include <initializer_list> // initializer_list
 #include <string> // char_traits, string
+#include <system_error> // errc
 #include <utility> // move
 #include <vector> // vector
 
+// #include <nlohmann/detail/conversions/from_chars.hpp>
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+
+
+// #include <nlohmann/detail/macro_scope.hpp>
+
+
+#if JSON_HAS_FROM_CHARS
+    #include <algorithm> // find, find_if
+    #include <charconv> // from_chars, from_chars_result
+    #include <cmath> // isnan
+#else
+    #include <cctype> // isspace
+    #include <cerrno> // ERANGE
+    #include <clocale> // localeconv, newlocale, freelocale
+    #include <cstdlib> // strto*
+    #include <memory> // unique_ptr
+    #include <type_traits> // enable_if, is_unsigned, remove_pointer
+    #include <utility> // declval
+
+    #ifdef __has_include
+        #if __has_include(<xlocale.h>)
+            #include <xlocale.h> // strto*_l, isspace_l
+        #endif
+    #endif
+#endif
+#include <limits> // numeric_limits<T>::[has_]infinity, quiet_NaN
+#include <system_error> // errc
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+
+constexpr char get_locale_independent_decimal_point() noexcept
+{
+    return '.';
+}
+
+#if JSON_HAS_FROM_CHARS
+
+//////////////////////////////////////////////
+// Delegate to std::from_chars if supported //
+//////////////////////////////////////////////
+
+/*!
+@brief Number parsing implementation details
+
+A traits class template that allows users of nlohmann::detail::from_chars to
+query details of the used implementation.
+
+@tparam T numeric type to parse, matching the `value` argument of from_chars
+*/
+template <typename T>
+struct from_chars_traits
+{
+    /// whether the from_chars in unaffected by the current global C locale
+    static constexpr bool is_locale_independent = true;
+
+    /// getter for the character used as decimal point by from_chars
+    static constexpr char(*get_decimal_point)() = &get_locale_independent_decimal_point;
+};
+
+using std::from_chars_result;
+
+/*!
+@brief Parse integer/floating-point numbers
+
+Parses the string [first, last) into the numeric type T.
+
+This implementation merely delegates to `std::from_chars` with one noteworthy
+difference: Different C++ standard library implementations do not agree on
+whether `value` should be set in the out-of-range case for floating-point
+numbers. Presently, libstdc++ leaves `value` unchanged whereas libc++ and MS STL
+set `value` to +/-0 or +/-inf which allows its users to distinguish between the
+various cases of over- and underflow. The C++ proposal [P4168][1] details this
+discrepancy and advocates to standardize the latter behavior.
+
+Since we need to be able to distinguish absolute underflow (+/-0) from absolute
+overflow (+/-inf), this implementation "corrects" the out-of-range behavior
+accordingly by estimating the effective exponent through manual string parsing.
+
+[1]: https://isocpp.org/files/papers/P4168R0.html
+
+@tparam T numeric type to parse
+@param[in] first points to the beginning of the parsed string (inclusive)
+@param[in] end points to the end of the parsed string (exclusive)
+@param[out] value references the variable to set the parsed number
+@return structure consisting of `ptr` and `ec` such that:
+        - If the string starting at `first` matches a number, `ptr` points to
+          the address immediately after the last character that matches.
+          * If the matched number can be represented by `T`, `value` is set and
+            `ec` is value-initialized.
+          * If the matched number cannot be represented by `T`, `ec` is set to
+            `std::errc::result_out_of_range` and depending on the type of `T`:
+            + `value` is left unchanged for integral `T`,
+            + `value` is set to +/-0 or +/-inf for floating-point `T`.
+        - If the string starting at `first` doesn't match a number, `ptr`
+          points to `first`, `ec` is `std::errc::invalid_argument`, and `value`
+          is left unchanged.
+*/
+template <typename T>
+JSON_HEDLEY_NON_NULL(1, 2)
+inline from_chars_result from_chars(const char* first, const char* last, T& value)
+{
+    // No implementation of std::from_chars will set its value argument to NaN,
+    // so by using NaN as the initial value, we can tell if it changed.
+    T inner_value = std::numeric_limits<T>::quiet_NaN();
+    auto res = std::from_chars(first, last, inner_value);
+
+    if constexpr (std::numeric_limits<T>::has_infinity)
+    {
+        if (res.ec == std::errc::result_out_of_range)
+        {
+            if (std::isnan(inner_value)) // inner_value was not set
+            {
+                // [first, res.ptr) matches a valid floating-point number that's
+                // out-of-range and our std::from_chars did not set `value`, so we
+                // need to distinguish the type of under-/overflow ourselves.
+                const char* mantissa_begin = *first == '-' ? first + 1 : first;
+                const char* mantissa_end = std::find_if(mantissa_begin, res.ptr, [](char c)
+                {
+                    return c == 'e' || c == 'E';
+                });
+                const char* decimal_point = std::find(mantissa_begin, mantissa_end, '.');
+                const char* significant_digit = std::find_if(mantissa_begin, mantissa_end, [](char d)
+                {
+                    return d >= '1' && d <= '9';
+                });
+
+                // Position of the significant digit relative to the decimal gives
+                // the order of magnitude of the mantissa.
+                auto effective_exponent = static_cast<int>(decimal_point - significant_digit);
+
+                // If the number includes an explicit exponent, add that to the the
+                // total exponent.
+                if (mantissa_end != res.ptr)
+                {
+                    const char* exponent_begin = *(mantissa_end + 1) == '+'
+                                                 ? mantissa_end + 2 // skip the exponent's + sign
+                                                 : mantissa_end + 1;
+                    int exponent = 0;
+                    auto exp_res = std::from_chars(exponent_begin, res.ptr, exponent);
+                    JSON_ASSERT(exp_res.ptr == res.ptr);
+
+                    if (exp_res.ec == std::errc::result_out_of_range)
+                    {
+                        // NOTE: Parentheses around function names mitigate min/max macro collision on Windows
+                        effective_exponent = *exponent_begin == '-'
+                                             ? (std::numeric_limits<int>::min)()
+                                             : (std::numeric_limits<int>::max)();
+                    }
+                    else
+                    {
+                        JSON_ASSERT(exp_res.ec == std::errc{});
+                        effective_exponent += exponent;
+                    }
+                }
+
+                // Set `value` to the sign-correct non-finite value based on the
+                // effective exponent.
+                value = (*first == '-' ? -1 : 1) * (effective_exponent < 0
+                                                    ? T{}
+                                                    : std::numeric_limits<T>::infinity());
+            }
+            else
+            {
+                value = inner_value;
+            }
+        }
+    }
+
+    if (res.ec == std::errc{})
+    {
+        value = inner_value;
+    }
+
+    return res;
+}
+
+#else
+
+//////////////////////////////////////////////
+// Fallback implementation using strto*[_l] //
+//////////////////////////////////////////////
+
+#if defined(_WIN32)
+
+/*!
+@brief Extended locale functions on Windows
+
+A trait object which provides wrappers for isspace and the strto* family of
+functions, based on the platform's support for extended locale APIs.
+
+This is the Windows implementation. The primary class template definition
+falls back to the standard locale-dependent functions, which is used on
+MinGW, whose C runtime only provides an incomplete subset of the
+`_<funcname>_l` family (e.g., missing `_strtof_l`/`_strtold_l` depending on
+the toolchain version). A specialization for genuine MSVC (including
+clang-cl, which mimics the MSVC ABI and CRT) is used instead, as the full
+`_<funcname>_l` family has reliably been available there since at least
+Visual Studio 2005.
+*/
+template <typename = const char*, typename = float>
+struct extended_locale_traits
+{
+    static constexpr bool is_locale_independent = false;
+
+    JSON_HEDLEY_PURE
+    static char get_decimal_point() noexcept
+    {
+        const auto* loc = localeconv();
+        JSON_ASSERT(loc != nullptr);
+        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
+    }
+
+    static int isspace(int c) noexcept
+    {
+        return std::isspace(c);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return std::strtoll(str, endptr, 10);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return std::strtoull(str, endptr, 10);
+    }
+
+    static constexpr float(&strtof)(const char*, char**) = std::strtof;
+    static constexpr double(&strtod)(const char*, char**) = std::strtod;
+    static constexpr long double(&strtold)(const char*, char**) = std::strtold;
+};
+
+#if defined(_MSC_VER)
+
+/*!
+@brief Get a pointer to a globally shared instance of the "C" locale on Windows
+
+This function uses _create_locale/_free_locale to allocate a "C" locale instance
+which can be passed to extended locale APIs. This instance is created on first
+use and has static storage duration, such that it can be used for the duration
+of the program.
+*/
+inline _locale_t get_c_locale_t() noexcept
+{
+    struct LocaleTDeleter
+    {
+        void operator()(_locale_t loc) noexcept
+        {
+            ::_free_locale(loc);
+        }
+    };
+    using LocaleUPtr = std::unique_ptr<std::remove_pointer<_locale_t>::type, LocaleTDeleter>;
+    static const LocaleUPtr c_locale = LocaleUPtr{::_create_locale(LC_ALL, "C"), LocaleTDeleter{}};
+    return c_locale.get();
+}
+
+template <typename T>
+struct extended_locale_traits<T, float>
+{
+    static constexpr bool is_locale_independent = true;
+    static constexpr char(&get_decimal_point)() = get_locale_independent_decimal_point;
+
+    static int isspace(int c) noexcept
+    {
+        return _isspace_l(c, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return _strtoll_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return _strtoull_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    static float strtof(T str, char** str_end)
+    {
+        return _strtof_l(str, str_end, get_c_locale_t());
+    }
+
+    static double strtod(T str, char** str_end)
+    {
+        return _strtod_l(str, str_end, get_c_locale_t());
+    }
+
+    static long double strtold(T str, char** str_end)
+    {
+        return _strtold_l(str, str_end, get_c_locale_t());
+    }
+};
+
+#endif
+
+#else
+
+/*!
+@brief Get a pointer to a globally shared instance of the "C" locale on POSIX
+
+This function uses newlocale/freelocale to allocate a "C" locale instance
+which can be passed to extended locale APIs. This instance is created on first
+use and has static storage duration, such that it can be used for the duration
+of the program.
+
+newlocale/freelocale themselves are in POSIX and are available independent of
+the platform's support for extended locale APIs.
+*/
+inline locale_t get_c_locale_t() noexcept
+{
+    struct LocaleTDeleter
+    {
+        void operator()(locale_t loc) noexcept
+        {
+            ::freelocale(loc);
+        }
+    };
+    using LocaleUPtr = std::unique_ptr<std::remove_pointer<locale_t>::type, LocaleTDeleter>;
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wexit-time-destructors"
+#endif
+    static const LocaleUPtr c_locale = LocaleUPtr {::newlocale(LC_ALL_MASK, "C", nullptr), LocaleTDeleter{}};
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
+    return c_locale.get();
+}
+
+/*!
+@brief Extended locale functions on POSIX
+
+A trait object which provides wrappers for isspace and the strto* family of
+functions, based on the platform's support for extended locale APIs.
+
+This is the POSIX implementation where extended locale support might not be
+available. The primary class template definition falls back to the standard
+locale-dependent functions. A partial specialization uses SFINAE to detect the
+availability of `strtof_l` (as a proxy for the whole `<funcname>_l` family) and
+provides access to locale-independent functions.
+*/
+template <typename = const char*, typename = float>
+struct extended_locale_traits
+{
+    static constexpr bool is_locale_independent = false;
+
+    /*!
+    @brief Query the decimal point used by the current C locale
+
+    Note that calling this function while switching the global C locale from
+    another thread is undefined behavior.
+    */
+    JSON_HEDLEY_PURE
+    static char get_decimal_point() noexcept
+    {
+        const auto* loc = localeconv();
+        JSON_ASSERT(loc != nullptr);
+        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
+    }
+
+    static int isspace(int c) noexcept
+    {
+        return std::isspace(c);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return std::strtoll(str, endptr, 10);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return std::strtoull(str, endptr, 10);
+    }
+
+    static constexpr float(&strtof)(const char*, char**) = std::strtof;
+    static constexpr double(&strtod)(const char*, char**) = std::strtod;
+    static constexpr long double(&strtold)(const char*, char**) = std::strtold;
+};
+
+template <typename T>
+struct extended_locale_traits<T, decltype(strtof_l(
+            std::declval<T>(),
+            std::declval<char**>(),
+            std::declval<locale_t>()))>
+{
+    static constexpr bool is_locale_independent = true;
+    static constexpr char(&get_decimal_point)() = get_locale_independent_decimal_point;
+
+    static int isspace(int c) noexcept
+    {
+        return isspace_l(c, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return ::strtoll_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return ::strtoull_l(str, endptr, 10, get_c_locale_t());
+    }
+
+    static float strtof(T str, char** str_end)
+    {
+        return ::strtof_l(str, str_end, get_c_locale_t());
+    }
+
+    static double strtod(T str, char** str_end)
+    {
+        return ::strtod_l(str, str_end, get_c_locale_t());
+    }
+
+    static long double strtold(T str, char** str_end)
+    {
+        return ::strtold_l(str, str_end, get_c_locale_t());
+    }
+};
+
+#endif
+
+/*!
+@brief Number parsing implementation details
+
+A traits class template that allows users of nlohmann::detail::from_chars to
+query details of the used implementation.
+
+Each specialization provides the following static members:
+- is_locale_independent: whether the from_chars in unaffected by the current
+                         global C locale
+- get_decimal_point: getter for the character used as decimal point by from_chars
+- strto: dispatches to the C standard library strto* function for type T, or to
+         the corresponding extended locale function, if available.
+
+@tparam T numeric type to parse, matching the `value` argument of from_chars
+*/
+template <typename T>
+struct from_chars_traits;
+
+template <>
+struct from_chars_traits<long long> // NOLINT(runtime/int)
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static constexpr long long(*strto)(const char*, char**) = &extended_locale_traits<>::strtoll;
+};
+
+template <>
+struct from_chars_traits<unsigned long long> // NOLINT(runtime/int)
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static constexpr unsigned long long(*strto)(const char*, char**) = &extended_locale_traits<>::strtoull;
+};
+
+template <>
+struct from_chars_traits<float>
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    static constexpr float(*strto)(const char*, char**) = &extended_locale_traits<>::strtof;
+};
+
+template <>
+struct from_chars_traits<double>
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    static constexpr double(*strto)(const char*, char**) = &extended_locale_traits<>::strtod;
+};
+
+template <>
+struct from_chars_traits<long double>
+{
+    static constexpr bool is_locale_independent = extended_locale_traits<>::is_locale_independent;
+
+    static char get_decimal_point()
+    {
+        return extended_locale_traits<>::get_decimal_point();
+    }
+
+    static constexpr long double(*strto)(const char*, char**) = &extended_locale_traits<>::strtold;
+};
+
+template <typename T>
+constexpr typename std::enable_if<std::numeric_limits<T>::has_infinity, bool>::type is_out_of_range_value(T value) noexcept
+{
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wfloat-equal"
+#endif
+    return value == T {} || value == std::numeric_limits<T>::infinity() || value == -std::numeric_limits<T>::infinity();
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
+}
+
+template <typename T>
+constexpr typename std::enable_if < !std::numeric_limits<T>::has_infinity, bool >::type is_out_of_range_value(T value) noexcept
+{
+    // NOTE: Parentheses around function names mitigate min/max macro collision on Windows
+    return value == (std::numeric_limits<T>::max)() || value == (std::numeric_limits<T>::min)();
+}
+
+struct from_chars_result
+{
+    const char* ptr;
+    std::errc ec;
+};
+
+/*!
+@brief Parse integer/floating-point numbers
+
+Parses the string [first, last) into the numeric type T.
+
+This implementation uses the C standard library functions strto*, or their
+extended locale counterparts strto*_l, to emulate the behavior of
+`std::from_chars` on platforms where it is not (fully) supported.
+
+Regarding the under-/overflow behavior, this implementation adopts the behavior
+proposed by [P4168][1] and implemented by libc++ and MS STL of setting `value`
+to the corresponding non-finite floating-point value in case of an out-of-range
+result.
+
+[1]: https://isocpp.org/files/papers/P4168R0.html
+
+@pre Unlike std::from_chars, this implementation requires the string to be
+     null-terminated, such that `last` dereferences into a NUL byte.
+
+@tparam T numeric type to parse
+@param[in] first points to the beginning of the parsed string (inclusive)
+@param[in] end points to the end of the parsed string (exclusive)
+@param[out] value references the variable to set the parsed number
+@return structure consisting of `ptr` and `ec` such that:
+        - If the string starting at `first` matches a number, `ptr` points to
+          the address immediately after the last character that matches.
+          * If the matched number can be represented by `T`, `value` is set and
+            `ec` is value-initialized.
+          * If the matched number cannot be represented by `T`, `ec` is set to
+            `std::errc::result_out_of_range` and depending on the type of `T`:
+            + `value` is left unchanged for integral `T`,
+            + `value` is set to +/-0 or +/-inf for floating-point `T`.
+        - If the string starting at `first` doesn't match a number, `ptr`
+          points to `first`, `ec` is `std::errc::invalid_argument`, and `value`
+          is left unchanged.
+*/
+template <typename T>
+JSON_HEDLEY_NON_NULL(1, 2)
+inline from_chars_result from_chars(const char* first, const char* last, T& value)
+{
+    JSON_ASSERT(*last == '\0');
+
+    // Unlike strto*, from_chars does not accept leading whitespace or + signs
+    if (first == last || *first == '+'
+            || (std::is_unsigned<T>::value && *first == '-')
+            || extended_locale_traits<>::isspace(*first) != 0)
+    {
+        return {first, std::errc::invalid_argument};
+    }
+
+    errno = 0;
+    char* ptr = nullptr; // NOLINT(misc-const-correctness)
+    T result = from_chars_traits<T>::strto(first, &ptr);
+
+    if (ptr == first)
+    {
+        return {ptr, std::errc::invalid_argument};
+    }
+
+    // Upon under-/overflow, strto* returns a marginal value and sets errno.
+    // Note that it is NOT sufficient to just check errno: strto* only clears
+    // errno if the parsed string actually parses into 0/[U]LLONG_MIN/-_MAX
+    // and this result was returned without indicating an under-/overflow.
+    // Otherwise, errno may not be relied upon to indicate the *absence* of
+    // an out-of-range error.
+    if (is_out_of_range_value(result) && errno == ERANGE)
+    {
+        if (std::numeric_limits<T>::has_infinity)
+        {
+            // ONLY for floating-point types, set `value` to the same non-finite
+            // result returned by strto* to allow users to distinguish between
+            // different types of under-/overflow.
+            value = result;
+        }
+        return {ptr, std::errc::result_out_of_range};
+    }
+
+    value = result;
+    return {ptr, std::errc{}};
+}
+
+#endif
+
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END
+
 // #include <nlohmann/detail/input/input_adapters.hpp>
 
 // #include <nlohmann/detail/input/position_t.hpp>
@@ -7837,7 +8489,7 @@ class lexer : public lexer_base<BasicJsonType>
     explicit lexer(InputAdapterType&& adapter, bool ignore_comments_ = false) noexcept
         : ia(std::move(adapter))
         , ignore_comments(ignore_comments_)
-        , decimal_point_char(static_cast<char_int_type>(get_decimal_point()))
+        , decimal_point_char(static_cast<char_int_type>(from_chars_traits<number_float_t>::get_decimal_point()))
     {}
 
     // deleted because of pointer members
@@ -7848,19 +8500,6 @@ class lexer : public lexer_base<BasicJsonType>
     ~lexer() = default;
 
   private:
-    /////////////////////
-    // locales
-    /////////////////////
-
-    /// return the locale-dependent decimal point
-    JSON_HEDLEY_PURE
-    static char get_decimal_point() noexcept
-    {
-        const auto* loc = localeconv();
-        JSON_ASSERT(loc != nullptr);
-        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
-    }
-
     /////////////////////
     // scan functions
     /////////////////////
@@ -8626,24 +9265,6 @@ class lexer : public lexer_base<BasicJsonType>
         }
     }
 
-    JSON_HEDLEY_NON_NULL(2)
-    static void strtof(float& f, const char* str, char** endptr) noexcept
-    {
-        f = std::strtof(str, endptr);
-    }
-
-    JSON_HEDLEY_NON_NULL(2)
-    static void strtof(double& f, const char* str, char** endptr) noexcept
-    {
-        f = std::strtod(str, endptr);
-    }
-
-    JSON_HEDLEY_NON_NULL(2)
-    static void strtof(long double& f, const char* str, char** endptr) noexcept
-    {
-        f = std::strtold(str, endptr);
-    }
-
     /*!
     @brief scan a number literal
 
@@ -8964,19 +9585,18 @@ scan_number_done:
         // we are done scanning a number)
         unget();
 
-        char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
-        errno = 0;
-
         // try to parse integers first and fall back to floats
         if (number_type == token_type::value_unsigned)
         {
-            const auto x = std::strtoull(token_buffer.data(), &endptr, 10);
+            unsigned long long x{}; // NOLINT(runtime/int)
+            const auto res = ::nlohmann::detail::from_chars(token_buffer.data(), token_buffer.data() + token_buffer.size(), x);
 
             // we checked the number format before
-            JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
+            JSON_ASSERT(res.ptr == token_buffer.data() + token_buffer.size());
 
-            if (errno != ERANGE)
+            if (res.ec != std::errc::result_out_of_range)
             {
+                JSON_ASSERT(res.ec == std::errc{});
                 value_unsigned = static_cast<number_unsigned_t>(x);
                 if (value_unsigned == x)
                 {
@@ -8986,13 +9606,15 @@ scan_number_done:
         }
         else if (number_type == token_type::value_integer)
         {
-            const auto x = std::strtoll(token_buffer.data(), &endptr, 10);
+            long long x{}; // NOLINT(runtime/int)
+            const auto res = ::nlohmann::detail::from_chars(token_buffer.data(), token_buffer.data() + token_buffer.size(), x);
 
             // we checked the number format before
-            JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
+            JSON_ASSERT(res.ptr == token_buffer.data() + token_buffer.size());
 
-            if (errno != ERANGE)
+            if (res.ec != std::errc::result_out_of_range)
             {
+                JSON_ASSERT(res.ec == std::errc{});
                 value_integer = static_cast<number_integer_t>(x);
                 if (value_integer == x)
                 {
@@ -9003,10 +9625,11 @@ scan_number_done:
 
         // this code is reached if we parse a floating-point number or if an
         // integer conversion above failed
-        strtof(value_float, token_buffer.data(), &endptr);
+        const auto res = ::nlohmann::detail::from_chars(token_buffer.data(), token_buffer.data() + token_buffer.size(), value_float);
 
         // we checked the number format before
-        JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
+        JSON_ASSERT(res.ptr == token_buffer.data() + token_buffer.size());
+        JSON_ASSERT(res.ec != std::errc::invalid_argument);
 
         return token_type::value_float;
     }
@@ -26555,6 +27178,7 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
     #undef JSON_HAS_CPP_26
     #undef JSON_HAS_FILESYSTEM
     #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
+    #undef JSON_HAS_FROM_CHARS
     #undef JSON_HAS_THREE_WAY_COMPARISON
     #undef JSON_HAS_RANGES
     #undef JSON_HAS_STD_FORMAT

tests/CMakeLists.txt

@@ -121,6 +121,10 @@ json_test_set_test_options(test-disabled_exceptions
 # raise timeout of expensive Unicode test
 json_test_set_test_options(test-unicode4 TEST_PROPERTIES TIMEOUT 3000)
 
+# link pthreads to tests that need it
+find_package(Threads REQUIRED)
+json_test_set_test_options(test-regression2 LINK_LIBRARIES Threads::Threads)
+
 #############################################################################
 # add unit tests
 #############################################################################

tests/src/unit-class_lexer.cpp

@@ -55,6 +55,8 @@ TEST_CASE("lexer class")
 
         SECTION("numbers")
         {
+            // Number parsing implementation uses std::from_chars where available,
+            // so run this test suite with JSON_HAS_CPP_17 as well.
             CHECK((scan_string("0") == json::lexer::token_type::value_unsigned));
             CHECK((scan_string("1") == json::lexer::token_type::value_unsigned));
             CHECK((scan_string("2") == json::lexer::token_type::value_unsigned));
@@ -65,13 +67,20 @@ TEST_CASE("lexer class")
             CHECK((scan_string("7") == json::lexer::token_type::value_unsigned));
             CHECK((scan_string("8") == json::lexer::token_type::value_unsigned));
             CHECK((scan_string("9") == json::lexer::token_type::value_unsigned));
+            CHECK((scan_string("18446744073709551615") == json::lexer::token_type::value_unsigned));
 
             CHECK((scan_string("-0") == json::lexer::token_type::value_integer));
             CHECK((scan_string("-1") == json::lexer::token_type::value_integer));
+            CHECK((scan_string("-9223372036854775808") == json::lexer::token_type::value_integer));
 
             CHECK((scan_string("1.1") == json::lexer::token_type::value_float));
             CHECK((scan_string("-1.1") == json::lexer::token_type::value_float));
             CHECK((scan_string("1E10") == json::lexer::token_type::value_float));
+
+            // out-of-range integers/floats are treated as value_float tokens
+            CHECK((scan_string("18446744073709551616") == json::lexer::token_type::value_float));
+            CHECK((scan_string("-9223372036854775809") == json::lexer::token_type::value_float));
+            CHECK((scan_string("1E400") == json::lexer::token_type::value_float));
         }
 
         SECTION("whitespace")

tests/src/unit-from_chars.cpp

@@ -0,0 +1,278 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <limits>
+#include <string>
+#include <system_error>
+
+// nlohmann::details::from_chars is a wrapper around std::from_chars when it is
+// available (__cpp_lib_to_chars is defined), otherwise our fallback
+// implementation kicks in.
+//
+// Due to the incomplete implementation of this C++17 standard library feature
+// (for example, lack of support for long double even in current libc++),
+// JSON_HAS_CPP_17 does not guarantee that std::from_chars is available.
+// However, the reverse holds: If the test is compiled in C++11 mode, the
+// fallback implementation will be used for sure.
+// By mentioning the JSON_HAS_CPP_17 macro in this here comment, the test will
+// be compiled both using our fallback and, at least on some platforms,
+// the C++17 standard library version, as a way of ensuring that the
+// expectations formulated in this test align with `std::from_chars`.
+
+namespace
+{
+struct init_val_t {};
+constexpr init_val_t init_val{};
+
+template <typename T>
+void check_result(std::string& str, T& value, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+{
+    // On platforms where neither `std::from_chars`, nor extended locale support (`strtof_l`) are
+    // available, the `strtof`-based implementation is in fact locale-dependent and might use a
+    // different decimal separator, so we need to adapt the test expectations accordingly.
+    std::replace(str.begin(), str.end(), '.', nlohmann::detail::from_chars_traits<T>::get_decimal_point());
+
+    auto res = nlohmann::detail::from_chars(str.data(), str.data() + str.size(), value);
+    CHECK_MESSAGE(res.ec == expected_ec, "Error code mismatch while parsing: \"", str,
+                  "\": Actual: ", make_error_code(res.ec).message(),
+                  "; expected: ", make_error_code(expected_ec).message());
+    CHECK_MESSAGE(res.ptr - str.data() == expected_ptr_offset, "Ptr offset mismatch while parsing: \"", str, "\"");
+}
+
+template <typename T>
+typename std::enable_if<std::is_integral<T>::value, void>::type
+check(std::string str, T expected_value, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+{
+    T value = 42;
+    check_result(str, value, expected_ptr_offset, expected_ec);
+    CHECK_MESSAGE(value == expected_value, "while parsing: \"", str, "\"");
+}
+
+template <typename T>
+typename std::enable_if<std::is_floating_point<T>::value, void>::type
+check(std::string str, T expected_value, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+{
+    {
+        T value = 42;
+        check_result(str, value, expected_ptr_offset, expected_ec);
+        CHECK_MESSAGE(value == expected_value, "while parsing: \"", str, "\"");
+        CHECK_MESSAGE(std::signbit(value) == std::signbit(expected_value), "while parsing: \"", str, "\"");
+    }
+    {
+        T value = std::numeric_limits<T>::quiet_NaN();
+        check_result(str, value, expected_ptr_offset, expected_ec);
+        CHECK_MESSAGE(value == expected_value, "while parsing: \"", str, "\"");
+    }
+}
+
+template <typename T>
+typename std::enable_if<std::is_integral<T>::value, void>::type
+check(std::string str, init_val_t /*unused*/, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+{
+    T const init_value = 42;
+    T value = init_value;
+    check_result(str, value, expected_ptr_offset, expected_ec);
+    CHECK_MESSAGE(value == init_value, "while parsing: \"", str, "\"");
+}
+
+template <typename T>
+typename std::enable_if<std::is_floating_point<T>::value, void>::type
+check(std::string str, init_val_t /*unused*/, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+{
+    {
+        T const init_value = 42;
+        T value = init_value;
+        check_result(str, value, expected_ptr_offset, expected_ec);
+        CHECK_MESSAGE(value == init_value, "while parsing: \"", str, "\"");
+    }
+    {
+        T value = std::numeric_limits<T>::quiet_NaN();
+        check_result(str, value, expected_ptr_offset, expected_ec);
+        CHECK_MESSAGE(doctest::IsNaN<T>(value), "while parsing: \"", str, "\"");
+    }
+}
+
+}  // namespace
+
+TEST_CASE("integral results consistent with std::from_chars")
+{
+    SECTION("unsigned long long")
+    {
+        check<unsigned long long>("", init_val, 0, std::errc::invalid_argument);
+        check<unsigned long long>("0", 0ULL, 1, std::errc{});
+        check<unsigned long long>(" 123", init_val, 0, std::errc::invalid_argument);
+        check<unsigned long long>("123 ", 123ULL, 3, std::errc{});
+        check<unsigned long long>("+123", init_val, 0, std::errc::invalid_argument);
+        check<unsigned long long>("-123", init_val, 0, std::errc::invalid_argument);
+        check<unsigned long long>("123", 123ULL, 3, std::errc{});
+        check<unsigned long long>("123e10", 123ULL, 3, std::errc{});
+        check<unsigned long long>("18446744073709551615", 18446744073709551615ULL, 20, std::errc{});
+        check<unsigned long long>("18446744073709551616", init_val, 20, std::errc::result_out_of_range);
+    }
+
+    SECTION("long long")
+    {
+        check<long long>("", init_val, 0, std::errc::invalid_argument);
+        check<long long>("0", 0LL, 1, std::errc{});
+        check<long long>(" 123", init_val, 0, std::errc::invalid_argument);
+        check<long long>("123 ", 123LL, 3, std::errc{});
+        check<long long>("+123", init_val, 0, std::errc::invalid_argument);
+        check<long long>("-123", -123LL, 4, std::errc{});
+        check<long long>("123", 123LL, 3, std::errc{});
+        check<long long>("123e10", 123LL, 3, std::errc{});
+        check<long long>("9223372036854775807", 9223372036854775807LL, 19, std::errc{});
+        check<long long>("9223372036854775808", init_val, 19, std::errc::result_out_of_range);
+        check<long long>("-9223372036854775808", -9223372036854775807LL - 1, 20, std::errc{});
+        check<long long>("-9223372036854775809", init_val, 20, std::errc::result_out_of_range);
+    }
+}
+
+TEST_CASE("floating point results consistent with std::from_chars")
+{
+    INFO("Using decimal point '",
+         std::string{1, nlohmann::detail::from_chars_traits<float>::get_decimal_point()},
+         "' to match our possibly-locale-dependent from_chars fallback implementation");
+
+    SECTION("single precision")
+    {
+        check<float>("", init_val, 0, std::errc::invalid_argument);
+        check<float>(" 123", init_val, 0, std::errc::invalid_argument);
+        check<float>("123 ", 123.0f, 3, std::errc{});
+        check<float>("+123", init_val, 0, std::errc::invalid_argument);
+        check<float>("-123", -123.0f, 4, std::errc{});
+        check<float>("123", 123.0f, 3, std::errc{});
+        check<float>("123e10", 123e10f, 6, std::errc{});
+        check<float>("123e+10", 123e10f, 7, std::errc{});
+        check<float>("123e-10", 123e-10f, 7, std::errc{});
+        check<float>("123.456", 123.456f, 7, std::errc{});
+        check<float>("123;456", 123.0f, 3, std::errc{});
+        check<float>("123.456 ", 123.456f, 7, std::errc{});
+        check<float>("123;456 ", 123.0f, 3, std::errc{});
+        check<float>("123456789.123456789", 123456789.123456789f, 19, std::errc{});
+        check<float>("1e40", std::numeric_limits<float>::infinity(), 4, std::errc::result_out_of_range);
+        check<float>("-1e40", -std::numeric_limits<float>::infinity(), 5, std::errc::result_out_of_range);
+        check<float>("2e308", std::numeric_limits<float>::infinity(), 5, std::errc::result_out_of_range);
+        check<float>("-2e308", -std::numeric_limits<float>::infinity(), 6, std::errc::result_out_of_range);
+        check<float>("123.456e-789", 0.0f, 12, std::errc::result_out_of_range);
+        check<float>("-123.456e-789", -0.0f, 13, std::errc::result_out_of_range);
+        check<float>("1e-45", 1e-45f, 5, std::errc{});
+        check<float>("1e-46", 0.0f, 5, std::errc::result_out_of_range);
+        check<float>("1E-45", 1e-45f, 5, std::errc{});
+        check<float>("1E-46", 0.0f, 5, std::errc::result_out_of_range);
+        check<float>("10e-46", 1e-45f, 6, std::errc{});
+        check<float>("0.1e-45", 0.0f, 7, std::errc::result_out_of_range);
+        check<float>("100000000000000000000000000000000000000", 1e38f, 39, std::errc{});
+        check<float>("100000000000000000000000000000000000000.0", 1e38f, 41, std::errc{});
+        check<float>("1000000000000000000000000000000000000000e-1", 1e38f, 43, std::errc{});
+        check<float>("0.0000000000000000000000000000000000000000000001e84", 1e38f, 51, std::errc{});
+        check<float>("0.000000000000000000000000000000000000000000001", 1e-45f, 47, std::errc{});
+        check<float>("00.000000000000000000000000000000000000000000001", 1e-45f, 48, std::errc{});
+        check<float>("0.0000000000000000000000000000000000000000000001e1", 1e-45f, 50, std::errc{});
+        check<float>("1000000000000000000000000000000000000000e-84", 1e-45f, 44, std::errc{});
+        check<float>("1000000000000000000000000000000000000000", std::numeric_limits<float>::infinity(), 40, std::errc::result_out_of_range);
+        check<float>("1000000000000000000000000000000000000000.0", std::numeric_limits<float>::infinity(), 42, std::errc::result_out_of_range);
+        check<float>("100000000000000000000000000000000000000e1", std::numeric_limits<float>::infinity(), 41, std::errc::result_out_of_range);
+        check<float>("0.0000000000000000000000000000000000000000000001e85", std::numeric_limits<float>::infinity(), 51, std::errc::result_out_of_range);
+        check<float>("1000000000000000000000000000000000000000e-85", 0.0f, 44, std::errc::result_out_of_range);
+        check<float>("0.0000000000000000000000000000000000000000000001", 0.0f, 48, std::errc::result_out_of_range);
+        check<float>("0.000000000000000000000000000000000000000000001e-1", 0.0f, 50, std::errc::result_out_of_range);
+        check<float>("1e-99999999999999999999", 0.0f, 23, std::errc::result_out_of_range);
+        check<float>("1e+99999999999999999999", std::numeric_limits<float>::infinity(), 23, std::errc::result_out_of_range);
+        check<float>("-1e-99999999999999999999", -0.0f, 24, std::errc::result_out_of_range);
+        check<float>("-1e+99999999999999999999", -std::numeric_limits<float>::infinity(), 24, std::errc::result_out_of_range);
+    }
+
+    SECTION("double precision")
+    {
+        check<double>("", init_val, 0, std::errc::invalid_argument);
+        check<double>(" 123", init_val, 0, std::errc::invalid_argument);
+        check<double>("123 ", 123.0, 3, std::errc{});
+        check<double>("+123", init_val, 0, std::errc::invalid_argument);
+        check<double>("-123", -123.0, 4, std::errc{});
+        check<double>("123", 123.0, 3, std::errc{});
+        check<double>("123e10", 123e10, 6, std::errc{});
+        check<double>("123e+10", 123e10, 7, std::errc{});
+        check<double>("123e-10", 123e-10, 7, std::errc{});
+        check<double>("123.456", 123.456, 7, std::errc{});
+        check<double>("123;456", 123.0, 3, std::errc{});
+        check<double>("123.456 ", 123.456, 7, std::errc{});
+        check<double>("123;456 ", 123.0, 3, std::errc{});
+        check<double>("123456789.123456789", 123456789.123456789, 19, std::errc{});
+        check<double>("1e40", 1e40, 4, std::errc{});
+        check<double>("-1e40", -1e40, 5, std::errc{});
+        check<double>("2e308", std::numeric_limits<double>::infinity(), 5, std::errc::result_out_of_range);
+        check<double>("-2e308", -std::numeric_limits<double>::infinity(), 6, std::errc::result_out_of_range);
+        check<double>("2e-324", 0.0, 6, std::errc::result_out_of_range);
+        check<double>("-2e-324", -0.0, 7, std::errc::result_out_of_range);
+        check<double>("123.456e-789", 0.0, 12, std::errc::result_out_of_range);
+        check<double>("-123.456e-789", -0.0, 13, std::errc::result_out_of_range);
+        check<double>("1e-99999999999999999999", 0.0, 23, std::errc::result_out_of_range);
+        check<double>("1e+99999999999999999999", std::numeric_limits<double>::infinity(), 23, std::errc::result_out_of_range);
+        check<double>("-1e-99999999999999999999", -0.0, 24, std::errc::result_out_of_range);
+        check<double>("-1e+99999999999999999999", -std::numeric_limits<double>::infinity(), 24, std::errc::result_out_of_range);
+    }
+
+    SECTION("long double precision")
+    {
+        check<long double>("", init_val, 0, std::errc::invalid_argument);
+        check<long double>(" 123", init_val, 0, std::errc::invalid_argument);
+        check<long double>("123 ", 123.0L, 3, std::errc{});
+        check<long double>("+123", init_val, 0, std::errc::invalid_argument);
+        check<long double>("-123", -123.0L, 4, std::errc{});
+        check<long double>("123", 123.0L, 3, std::errc{});
+        check<long double>("123e10", 123e10L, 6, std::errc{});
+        check<long double>("123e+10", 123e10L, 7, std::errc{});
+        check<long double>("123e-10", 123e-10L, 7, std::errc{});
+        check<long double>("123.456", 123.456L, 7, std::errc{});
+        check<long double>("123;456", 123.0L, 3, std::errc{});
+        check<long double>("123.456 ", 123.456L, 7, std::errc{});
+        check<long double>("123;456 ", 123.0L, 3, std::errc{});
+        check<long double>("123456789.123456789", 123456789.123456789L, 19, std::errc{});
+        check<long double>("1e40", 1e40L, 4, std::errc{});
+        check<long double>("-1e40", -1e40L, 5, std::errc{});
+#if defined(_MSC_VER)
+#pragma warning(push)
+#pragma warning(disable: 4127)
+#endif
+        if (sizeof(long double) > 8)
+#if defined(_MSC_VER)
+#pragma warning(pop)
+#endif
+        {
+            // Right-hand-side is calculated to avoid warning about literal
+            // exceeding range on platforms where this branch is NOT taken.
+            check<long double>("2e308", 1e308L * 2, 5, std::errc{});
+            check<long double>("-2e308", -1e308L * 2, 6, std::errc{});
+            check<long double>("2e-324", 8e-324L / 4, 6, std::errc{});
+            check<long double>("-2e-324", -8e-324L / 4, 7, std::errc{});
+        }
+        else
+        {
+            check<long double>("2e308", std::numeric_limits<long double>::infinity(), 5, std::errc::result_out_of_range);
+            check<long double>("-2e308", -std::numeric_limits<long double>::infinity(), 6, std::errc::result_out_of_range);
+            check<long double>("2e-324", 0.0L, 6, std::errc::result_out_of_range);
+            check<long double>("-2e-324", -0.0L, 7, std::errc::result_out_of_range);
+        }
+        check<long double>("1e5000", std::numeric_limits<long double>::infinity(), 6, std::errc::result_out_of_range);
+        check<long double>("-1e5000", -std::numeric_limits<long double>::infinity(), 7, std::errc::result_out_of_range);
+        check<long double>("123.456e-7890", 0.0L, 13, std::errc::result_out_of_range);
+        check<long double>("-123.456e-7890", -0.0L, 14, std::errc::result_out_of_range);
+        check<long double>("1e-99999999999999999999", 0.0L, 23, std::errc::result_out_of_range);
+        check<long double>("1e+99999999999999999999", std::numeric_limits<long double>::infinity(), 23, std::errc::result_out_of_range);
+        check<long double>("-1e-99999999999999999999", -0.0L, 24, std::errc::result_out_of_range);
+        check<long double>("-1e+99999999999999999999", -std::numeric_limits<long double>::infinity(), 24, std::errc::result_out_of_range);
+    }
+}
+

tests/src/unit-regression2.cpp

@@ -26,8 +26,11 @@ using ordered_json = nlohmann::ordered_json;
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
+#include <atomic>
+#include <clocale>
 #include <cstdio>
 #include <list>
+#include <thread>
 #include <type_traits>
 #include <utility>
 
@@ -1239,6 +1242,44 @@ TEST_CASE("regression tests 2")
         CHECK(j == json({2, 4, 6}));
     }
 #endif
+
+    SECTION("issue #5198 - TOCTOU race between lexer construction and locale changes causes float truncation")
+    {
+        std::atomic<bool> stop_requested{};
+        std::thread switcher([&stop_requested]
+        {
+            bool german = true;
+            const std::string initial_locale = std::setlocale(LC_NUMERIC, nullptr);
+
+            while (!stop_requested)
+            {
+                const bool setlocale_res = std::setlocale(LC_NUMERIC, german ? "de_DE.UTF-8" : "C");
+                WARN_MESSAGE(setlocale_res,
+                             "Setting locale failed, probably because de_DE.UTF-8 is not installed. "
+                             "This test was skipped as it would be inconclusive.");
+                if (!setlocale_res)
+                {
+                    stop_requested = true;
+                }
+                german = !german;
+            }
+
+            (void)std::setlocale(LC_NUMERIC, initial_locale.c_str()); // restore original locale
+        });
+
+        for (std::size_t i = 0; i < 10000 && !stop_requested; ++i)
+        {
+            const json j = json::parse("{\"val\": 99.123456789}");
+            const double parsed = j.value("val", 0.0);
+            if (!CHECK(parsed == 99.123456789))
+            {
+                break;
+            }
+        }
+
+        stop_requested = true;
+        switcher.join();
+    }
 }
 
 TEST_CASE_TEMPLATE("issue #4798 - nlohmann::json::to_msgpack() encode float NaN as double", T, double, float) // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)