Extend std::optional null regression coverage (assignment, get_to) (#5269)

Both had zero documentation anywhere in docs/mkdocs/. The tuple/pair
gap was first spotted in the very first git-log audit pass but never
turned into an actionable todo, so it persisted uncaught across four
subsequent passes.

- Document basic positional std::pair/std::tuple <-> json array
  conversion, plus #5016's reference-extraction capability
  (get<std::tuple<T&, T&>>() returning references into the stored
  array elements).
- Document #5205's new json-from-C++20-range-view constructor
  (e.g. nums | std::views::filter(...)).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/binary_t.md

@@ -51,6 +51,38 @@ represent a byte array in modern C++.
 
 The default values for `BinaryType` is `#!cpp std::vector<std::uint8_t>`.
 
+#### Custom BinaryType behavior
+
+When a custom `BinaryType` is configured (other than the default `#!cpp std::vector<std::uint8_t>`), you can assign
+values of that type directly to a `basic_json` instance, and they will automatically be recognized as binary values
+rather than arrays:
+
+```cpp
+using custom_json = nlohmann::basic_json<
+    nlohmann::ordered_map,  // ObjectType
+    std::vector,            // ArrayType
+    std::string,            // StringType
+    bool,                   // BooleanType
+    std::int64_t,           // NumberIntegerType
+    std::uint64_t,          // NumberUnsignedType
+    double,                 // NumberFloatType
+    std::allocator,         // AllocatorType
+    nlohmann::adl_serializer,
+    std::vector<std::byte>  // Custom BinaryType
+>;
+
+std::vector<std::byte> data{std::byte{1}, std::byte{2}, std::byte{3}};
+custom_json j = data;  // Creates a binary value, not an array
+assert(j.is_binary());
+
+// Round-tripping works seamlessly
+auto extracted = j.get<std::vector<std::byte>>();
+assert(extracted == data);
+```
+
+This automatic type detection is a convenience feature that only applies to custom (non-default) `BinaryType` configurations.
+The default `nlohmann::json` continues to treat `#!cpp std::vector<std::uint8_t>` as arrays for backward compatibility.
+
 #### Storage
 
 Binary Arrays are stored as pointers in a `basic_json` type. That is, for any access to array values, a pointer of the

docs/mkdocs/docs/api/macros/json_diagnostics.md

@@ -38,7 +38,8 @@ When the macro is not defined, the library will define it to its default value.
 
     Diagnostic messages can also be controlled with the CMake option
     [`JSON_Diagnostics`](../../integration/cmake.md#json_diagnostics) (`OFF` by default)
-    which defines `JSON_DIAGNOSTICS` accordingly.
+    which defines `JSON_DIAGNOSTICS` accordingly. Note this only applies when building the
+    library from source — see the pre-installed-package caveat on that page.
 
 ## Examples
 

docs/mkdocs/docs/features/conversions.md

@@ -47,6 +47,28 @@ json j = {{"one", 1}, {"two", 2}};
 auto m = j.get<std::map<std::string, int>>();  // {{"one", 1}, {"two", 2}}
 ```
 
+`#!cpp std::pair` and `#!cpp std::tuple` are also supported, converting positionally to and from a JSON array:
+
+```cpp
+json j = {1.0, "hello", 42};
+auto t = j.get<std::tuple<double, std::string, int>>();  // {1.0, "hello", 42}
+```
+
+!!! info "Extracting references into a tuple"
+
+    A tuple type may also hold references (e.g. `#!cpp std::tuple<double&, std::string&>`) to avoid copying: `get`
+    then returns a tuple of references pointing directly at the elements stored inside the `basic_json` array,
+    rather than a tuple of copies:
+
+    ```cpp
+    json j = {1.0, "hello"};
+    auto refs = j.get<std::tuple<double&, std::string&>>();
+    std::get<1>(refs) = "world";  // modifies j[1] in place
+    ```
+
+    A referenced type must be one the library actually stores (or an arithmetic type it can convert to/from);
+    otherwise this is a compile error.
+
 ## Implicit conversions
 
 By default, a JSON value implicitly converts to a compatible C++ type, so the explicit `get` call can often be omitted:
@@ -136,6 +158,20 @@ std::vector<int> numbers = {1, 2, 3};
 json j = numbers;   // [1,2,3]
 ```
 
+!!! info "Constructing from a C++20 range view"
+
+    A `json` array can also be constructed directly from a C++20 range view (`std::ranges::view`), such as the result
+    of `std::views::filter` or `std::views::transform` -- no intermediate container is needed:
+
+    ```cpp
+    std::vector<int> nums{1, 2, 37, 42, 21};
+    auto filtered = nums | std::views::filter([](int i) { return i > 10; });
+    json j(filtered);   // [37,42,21]
+    ```
+
+    This requires [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md) to be enabled and is unavailable on MinGW due
+    to incomplete C++20 ranges support there.
+
 ## Your own types
 
 The conversions above are built in for standard types. To make the same syntax work for **your own** types, provide

docs/mkdocs/docs/integration/cmake.md

@@ -135,6 +135,31 @@ Enable CI build targets. The exact targets are used during the several CI steps
 
 Enable [extended diagnostic messages](../home/exceptions.md#extended-diagnostic-messages) by defining macro [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This option is `OFF` by default.
 
+!!! warning "Does not apply to a pre-installed package"
+
+    This option only takes effect when building nlohmann/json from source as part of your own
+    CMake project (e.g. via [`FetchContent`](#fetchcontent) or [`add_subdirectory`](#external)).
+    It has **no effect** on a package that was already built and installed elsewhere (Homebrew,
+    vcpkg, a system package, etc.) — the resulting compile definition is baked into the exported
+    `nlohmann_jsonTargets.cmake` at install time, and `set(JSON_Diagnostics ON)` before
+    `find_package()` does not change it (verified against the Homebrew-installed package: the
+    exported target still carries a fixed `$<$<BOOL:OFF>:JSON_DIAGNOSTICS=1>`, regardless of any
+    variable set in the consuming project).
+
+    To enable extended diagnostics for a pre-installed package, override the imported target's
+    property directly after `find_package()`:
+
+    ```cmake
+    find_package(nlohmann_json REQUIRED)
+    set_target_properties(nlohmann_json::nlohmann_json PROPERTIES
+        INTERFACE_COMPILE_DEFINITIONS "JSON_DIAGNOSTICS=1")
+    ```
+
+    This only works cleanly when your project is the sole consumer of that imported target. If
+    nlohmann_json is pulled in from more than one place in your dependency graph with different
+    `JSON_DIAGNOSTICS` values, you may see a `"JSON_DIAGNOSTICS" redefined` compiler error, since
+    conflicting `-D` flags can end up on the same compile command line.
+
 ### `JSON_Diagnostic_Positions`
 
 Enable position diagnostics by defining macro [`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md). This option is `OFF` by default.

include/nlohmann/detail/conversions/from_json.hpp

@@ -19,6 +19,7 @@
 #include <unordered_map> // unordered_map
 #include <utility> // pair, declval
 #include <valarray> // valarray
+#include <vector> // vector
 
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
@@ -332,6 +333,7 @@ template < typename BasicJsonType, typename ConstructibleArrayType,
                !is_constructible_object_type<BasicJsonType, ConstructibleArrayType>::value&&
                !is_constructible_string_type<BasicJsonType, ConstructibleArrayType>::value&&
                !std::is_same<ConstructibleArrayType, typename BasicJsonType::binary_t>::value&&
+               !is_compatible_binary_type<BasicJsonType, ConstructibleArrayType>::value&&
                !is_basic_json<ConstructibleArrayType>::value,
                int > = 0 >
 auto from_json(const BasicJsonType& j, ConstructibleArrayType& arr)
@@ -377,6 +379,25 @@ inline void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t&
     bin = *j.template get_ptr<const typename BasicJsonType::binary_t*>();
 }
 
+template < typename BasicJsonType, typename CompatibleArrayType,
+           enable_if_t < is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value,
+                         int > = 0 >
+inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
+{
+    if (j.is_binary())
+    {
+        bin = static_cast<CompatibleArrayType>(*j.template get_ptr<const typename BasicJsonType::binary_t*>());
+    }
+    else if (j.is_array())
+    {
+        from_json_array_impl(j, bin, priority_tag<3> {});
+    }
+    else
+    {
+        JSON_THROW(type_error::create(302, concat("type must be binary or array, but is ", j.type_name()), &j));
+    }
+}
+
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)

include/nlohmann/detail/conversions/to_json.hpp

@@ -177,8 +177,11 @@ struct external_constructor<value_t::array>
     }
 
     template < typename BasicJsonType, typename CompatibleArrayType,
-               enable_if_t < !std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value,
+               enable_if_t < !std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value
-                             int > = 0 >
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+                             && !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+                             , int > = 0 >
     static void construct(BasicJsonType& j, const CompatibleArrayType& arr)
     {
         using std::begin;
@@ -218,6 +221,25 @@ struct external_constructor<value_t::array>
         j.set_parents();
         j.assert_invariant();
     }
+
+    // std::ranges does not work properly on MinGW due to incomplete C++20 support
+    // see https://github.com/nlohmann/json/issues/4916
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+    template<typename BasicJsonType, typename CompatibleArrayType,
+             enable_if_t<is_compatible_range_view<std::remove_cvref_t<CompatibleArrayType>>::value, int> = 0>
+    static void construct(BasicJsonType& j, CompatibleArrayType && arr)
+    {
+        j.m_data.m_value.destroy(j.m_data.m_type);
+        j.m_data.m_type = value_t::array;
+        j.m_data.m_value = value_t::array;
+        for (auto&& x : std::forward<CompatibleArrayType>(arr))
+        {
+            j.m_data.m_value.array->push_back(x);
+            j.set_parent(j.m_data.m_value.array->back());
+        }
+        j.assert_invariant();
+    }
+#endif
 };
 
 template<>
@@ -355,19 +377,44 @@ template < typename BasicJsonType, typename CompatibleArrayType,
                          !is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value&&
                          !is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value&&
                          !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value&&
-                         !is_basic_json<CompatibleArrayType>::value,
+                         !is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value&&
+                         !is_basic_json<CompatibleArrayType>::value
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+    && !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+                         ,
                          int > = 0 >
 inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
 
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+template < typename BasicJsonType, typename T,
+           enable_if_t < is_compatible_range_view<std::remove_cvref_t<T>>::value
+                         && !is_compatible_string_type<BasicJsonType, std::remove_cvref_t<T>>::value
+                         && !is_compatible_object_type<BasicJsonType, std::remove_cvref_t<T>>::value
+                         && !is_basic_json<std::remove_cvref_t<T>>::value, int > = 0 >
+inline void to_json(BasicJsonType& j, T && arr)
+{
+    external_constructor<value_t::array>::construct(j, std::forward<T>(arr));
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bin)
 {
     external_constructor<value_t::binary>::construct(j, bin);
 }
 
+template < typename BasicJsonType, typename CompatibleArrayType,
+           enable_if_t < is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value,
+                         int > = 0 >
+inline void to_json(BasicJsonType& j, const CompatibleArrayType& bin)
+{
+    external_constructor<value_t::binary>::construct(j, typename BasicJsonType::binary_t(bin));
+}
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, const std::valarray<T>& arr)

include/nlohmann/detail/meta/type_traits.hpp

@@ -13,11 +13,15 @@
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
+#include <vector> // vector
 #if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
     #include <cstddef> // byte
 #endif
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
 #include <nlohmann/detail/meta/call_std/begin.hpp>
 #include <nlohmann/detail/meta/call_std/end.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
@@ -450,6 +454,51 @@ struct is_constructible_string_type
         value_type_t, laundered_type >>::value;
 };
 
+// Forward declarations: iteration_proxy.hpp includes this file, so we cannot
+// include it here.
+template<typename IteratorType> class iteration_proxy;
+template<typename IteratorType> class iteration_proxy_value;
+
+// Identifies nlohmann's internal iteration-proxy types. These must be excluded
+// before evaluating any std::ranges concept to avoid circular constraints.
+template<typename T> struct is_iteration_proxy_type : std::false_type {};
+template<typename T> struct is_iteration_proxy_type<iteration_proxy<T>>       : std::true_type {};
+template<typename T> struct is_iteration_proxy_type<iteration_proxy_value<T>> : std::true_type {};
+
+// In C++26, std::optional satisfies std::ranges::view; exclude it so the
+// range-view overload does not hijack the optional serializer.
+#ifdef JSON_HAS_CPP_17
+template<typename T> struct is_range_view_optional_type : std::false_type {};
+template<typename T> struct is_range_view_optional_type<std::optional<T>> : std::true_type {};
+#else
+template<typename T> struct is_range_view_optional_type : std::false_type {};
+#endif
+
+// std::ranges does not work properly on MinGW due to incomplete C++20 support
+// see https://github.com/nlohmann/json/issues/4916
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+
+// SafeToCheck guards against types that trigger circular constraints when
+// std::ranges::view<T> is evaluated on GCC 12 / libstdc++ 12:
+//   - iteration_proxy / iteration_proxy_value directly
+//   - views wrapping the above (e.g. owning_view<iteration_proxy<...>>)
+//   - views wrapping basic_json (e.g. ref_view<json>) — same circularity
+//     via json's constructors → is_compatible_array_type → here
+// nlohmann's plain range_value_t (iterator_traits-based) is safe to call
+// before any std::ranges concept is touched, so we use it for the checks.
+template < typename T, bool SafeToCheck =
+           !is_iteration_proxy_type<T>::value &&
+           !is_iteration_proxy_type<detected_t<range_value_t, T>>::value &&
+           !is_basic_json<detected_t<range_value_t, T>>::value &&
+           !is_range_view_optional_type<T>::value >
+struct is_compatible_range_view : std::false_type {};
+
+template<typename T>
+struct is_compatible_range_view<T, true>
+    : std::bool_constant<std::ranges::view<T>> {};
+
+#endif
+
 template<typename BasicJsonType, typename CompatibleArrayType, typename = void>
 struct is_compatible_array_type_impl : std::false_type {};
 
@@ -461,13 +510,38 @@ struct is_compatible_array_type_impl <
     is_iterator_traits<iterator_traits<detected_t<iterator_t, CompatibleArrayType>>>::value&&
 // special case for types like std::filesystem::path whose iterator's value_type are themselves
 // c.f. https://github.com/nlohmann/json/pull/3073
-    !std::is_same<CompatibleArrayType, detected_t<range_value_t, CompatibleArrayType>>::value >>
+    !std::is_same<CompatibleArrayType, detected_t<range_value_t, CompatibleArrayType>>::value
+// When range-view support is enabled, std::ranges::view types (e.g. std::string_view,
+// filter_view) can match BOTH this iterator-based specialization AND the view-based one
+// below, causing ambiguity. Exclude views here so the two specializations are mutually
+// exclusive: this one handles plain iterable containers, the other handles views.
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+&& !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+            >>
 {
     static constexpr bool value =
         is_constructible<BasicJsonType,
         range_value_t<CompatibleArrayType>>::value;
 };
 
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+template<typename BasicJsonType, typename CompatibleArrayType>
+struct is_compatible_array_type_impl <
+    BasicJsonType, CompatibleArrayType,
+    enable_if_t < is_compatible_range_view<CompatibleArrayType>::value
+    && !std::is_same<detected_t<range_value_t, CompatibleArrayType>, char>::value
+    && !std::is_same<detected_t<range_value_t, CompatibleArrayType>, wchar_t>::value >>
+{
+    // CompatibleArrayType is a std::ranges::view here, so std::ranges::range_value_t
+    // is safe and correctly handles C++20 iterators that may lack classic iterator_traits.
+    static constexpr bool value =
+        is_constructible<BasicJsonType,
+        std::ranges::range_value_t<CompatibleArrayType>>::value;
+};
+#endif
+
+
 template<typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_array_type
     : is_compatible_array_type_impl<BasicJsonType, CompatibleArrayType> {};
@@ -559,6 +633,14 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
+template<typename BasicJsonType, typename CompatibleArrayType>
+struct is_compatible_binary_type
+{
+    static constexpr bool value =
+        std::is_same<typename BasicJsonType::binary_t::container_type, CompatibleArrayType>::value &&
+        !std::is_same<typename BasicJsonType::binary_t::container_type, std::vector<std::uint8_t>>::value;
+};
+
 template<typename BasicJsonType, typename CompatibleReferenceType>
 struct is_compatible_reference_type_impl
 {

single_include/nlohmann/json.hpp

@@ -189,6 +189,7 @@
 #include <unordered_map> // unordered_map
 #include <utility> // pair, declval
 #include <valarray> // valarray
+#include <vector> // vector
 
 // #include <nlohmann/detail/exceptions.hpp>
 //     __ _____ _____ _____
@@ -3592,6 +3593,7 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
+#include <vector> // vector
 #if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
     #include <cstddef> // byte
 #endif
@@ -3663,6 +3665,9 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/macro_scope.hpp>
 
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
 // #include <nlohmann/detail/meta/call_std/begin.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
@@ -4212,6 +4217,51 @@ struct is_constructible_string_type
         value_type_t, laundered_type >>::value;
 };
 
+// Forward declarations: iteration_proxy.hpp includes this file, so we cannot
+// include it here.
+template<typename IteratorType> class iteration_proxy;
+template<typename IteratorType> class iteration_proxy_value;
+
+// Identifies nlohmann's internal iteration-proxy types. These must be excluded
+// before evaluating any std::ranges concept to avoid circular constraints.
+template<typename T> struct is_iteration_proxy_type : std::false_type {};
+template<typename T> struct is_iteration_proxy_type<iteration_proxy<T>>       : std::true_type {};
+template<typename T> struct is_iteration_proxy_type<iteration_proxy_value<T>> : std::true_type {};
+
+// In C++26, std::optional satisfies std::ranges::view; exclude it so the
+// range-view overload does not hijack the optional serializer.
+#ifdef JSON_HAS_CPP_17
+template<typename T> struct is_range_view_optional_type : std::false_type {};
+template<typename T> struct is_range_view_optional_type<std::optional<T>> : std::true_type {};
+#else
+template<typename T> struct is_range_view_optional_type : std::false_type {};
+#endif
+
+// std::ranges does not work properly on MinGW due to incomplete C++20 support
+// see https://github.com/nlohmann/json/issues/4916
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+
+// SafeToCheck guards against types that trigger circular constraints when
+// std::ranges::view<T> is evaluated on GCC 12 / libstdc++ 12:
+//   - iteration_proxy / iteration_proxy_value directly
+//   - views wrapping the above (e.g. owning_view<iteration_proxy<...>>)
+//   - views wrapping basic_json (e.g. ref_view<json>) — same circularity
+//     via json's constructors → is_compatible_array_type → here
+// nlohmann's plain range_value_t (iterator_traits-based) is safe to call
+// before any std::ranges concept is touched, so we use it for the checks.
+template < typename T, bool SafeToCheck =
+           !is_iteration_proxy_type<T>::value &&
+           !is_iteration_proxy_type<detected_t<range_value_t, T>>::value &&
+           !is_basic_json<detected_t<range_value_t, T>>::value &&
+           !is_range_view_optional_type<T>::value >
+struct is_compatible_range_view : std::false_type {};
+
+template<typename T>
+struct is_compatible_range_view<T, true>
+    : std::bool_constant<std::ranges::view<T>> {};
+
+#endif
+
 template<typename BasicJsonType, typename CompatibleArrayType, typename = void>
 struct is_compatible_array_type_impl : std::false_type {};
 
@@ -4223,13 +4273,38 @@ struct is_compatible_array_type_impl <
     is_iterator_traits<iterator_traits<detected_t<iterator_t, CompatibleArrayType>>>::value&&
 // special case for types like std::filesystem::path whose iterator's value_type are themselves
 // c.f. https://github.com/nlohmann/json/pull/3073
-    !std::is_same<CompatibleArrayType, detected_t<range_value_t, CompatibleArrayType>>::value >>
+    !std::is_same<CompatibleArrayType, detected_t<range_value_t, CompatibleArrayType>>::value
+// When range-view support is enabled, std::ranges::view types (e.g. std::string_view,
+// filter_view) can match BOTH this iterator-based specialization AND the view-based one
+// below, causing ambiguity. Exclude views here so the two specializations are mutually
+// exclusive: this one handles plain iterable containers, the other handles views.
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+&& !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+            >>
 {
     static constexpr bool value =
         is_constructible<BasicJsonType,
         range_value_t<CompatibleArrayType>>::value;
 };
 
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+template<typename BasicJsonType, typename CompatibleArrayType>
+struct is_compatible_array_type_impl <
+    BasicJsonType, CompatibleArrayType,
+    enable_if_t < is_compatible_range_view<CompatibleArrayType>::value
+    && !std::is_same<detected_t<range_value_t, CompatibleArrayType>, char>::value
+    && !std::is_same<detected_t<range_value_t, CompatibleArrayType>, wchar_t>::value >>
+{
+    // CompatibleArrayType is a std::ranges::view here, so std::ranges::range_value_t
+    // is safe and correctly handles C++20 iterators that may lack classic iterator_traits.
+    static constexpr bool value =
+        is_constructible<BasicJsonType,
+        std::ranges::range_value_t<CompatibleArrayType>>::value;
+};
+#endif
+
+
 template<typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_array_type
     : is_compatible_array_type_impl<BasicJsonType, CompatibleArrayType> {};
@@ -4321,6 +4396,14 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
+template<typename BasicJsonType, typename CompatibleArrayType>
+struct is_compatible_binary_type
+{
+    static constexpr bool value =
+        std::is_same<typename BasicJsonType::binary_t::container_type, CompatibleArrayType>::value &&
+        !std::is_same<typename BasicJsonType::binary_t::container_type, std::vector<std::uint8_t>>::value;
+};
+
 template<typename BasicJsonType, typename CompatibleReferenceType>
 struct is_compatible_reference_type_impl
 {
@@ -5464,6 +5547,7 @@ template < typename BasicJsonType, typename ConstructibleArrayType,
                !is_constructible_object_type<BasicJsonType, ConstructibleArrayType>::value&&
                !is_constructible_string_type<BasicJsonType, ConstructibleArrayType>::value&&
                !std::is_same<ConstructibleArrayType, typename BasicJsonType::binary_t>::value&&
+               !is_compatible_binary_type<BasicJsonType, ConstructibleArrayType>::value&&
                !is_basic_json<ConstructibleArrayType>::value,
                int > = 0 >
 auto from_json(const BasicJsonType& j, ConstructibleArrayType& arr)
@@ -5509,6 +5593,25 @@ inline void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t&
     bin = *j.template get_ptr<const typename BasicJsonType::binary_t*>();
 }
 
+template < typename BasicJsonType, typename CompatibleArrayType,
+           enable_if_t < is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value,
+                         int > = 0 >
+inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
+{
+    if (j.is_binary())
+    {
+        bin = static_cast<CompatibleArrayType>(*j.template get_ptr<const typename BasicJsonType::binary_t*>());
+    }
+    else if (j.is_array())
+    {
+        from_json_array_impl(j, bin, priority_tag<3> {});
+    }
+    else
+    {
+        JSON_THROW(type_error::create(302, concat("type must be binary or array, but is ", j.type_name()), &j));
+    }
+}
+
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -6205,8 +6308,11 @@ struct external_constructor<value_t::array>
     }
 
     template < typename BasicJsonType, typename CompatibleArrayType,
-               enable_if_t < !std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value,
+               enable_if_t < !std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value
-                             int > = 0 >
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+                             && !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+                             , int > = 0 >
     static void construct(BasicJsonType& j, const CompatibleArrayType& arr)
     {
         using std::begin;
@@ -6246,6 +6352,25 @@ struct external_constructor<value_t::array>
         j.set_parents();
         j.assert_invariant();
     }
+
+    // std::ranges does not work properly on MinGW due to incomplete C++20 support
+    // see https://github.com/nlohmann/json/issues/4916
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+    template<typename BasicJsonType, typename CompatibleArrayType,
+             enable_if_t<is_compatible_range_view<std::remove_cvref_t<CompatibleArrayType>>::value, int> = 0>
+    static void construct(BasicJsonType& j, CompatibleArrayType && arr)
+    {
+        j.m_data.m_value.destroy(j.m_data.m_type);
+        j.m_data.m_type = value_t::array;
+        j.m_data.m_value = value_t::array;
+        for (auto&& x : std::forward<CompatibleArrayType>(arr))
+        {
+            j.m_data.m_value.array->push_back(x);
+            j.set_parent(j.m_data.m_value.array->back());
+        }
+        j.assert_invariant();
+    }
+#endif
 };
 
 template<>
@@ -6383,19 +6508,44 @@ template < typename BasicJsonType, typename CompatibleArrayType,
                          !is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value&&
                          !is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value&&
                          !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value&&
-                         !is_basic_json<CompatibleArrayType>::value,
+                         !is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value&&
+                         !is_basic_json<CompatibleArrayType>::value
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+    && !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+                         ,
                          int > = 0 >
 inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
 
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+template < typename BasicJsonType, typename T,
+           enable_if_t < is_compatible_range_view<std::remove_cvref_t<T>>::value
+                         && !is_compatible_string_type<BasicJsonType, std::remove_cvref_t<T>>::value
+                         && !is_compatible_object_type<BasicJsonType, std::remove_cvref_t<T>>::value
+                         && !is_basic_json<std::remove_cvref_t<T>>::value, int > = 0 >
+inline void to_json(BasicJsonType& j, T && arr)
+{
+    external_constructor<value_t::array>::construct(j, std::forward<T>(arr));
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bin)
 {
     external_constructor<value_t::binary>::construct(j, bin);
 }
 
+template < typename BasicJsonType, typename CompatibleArrayType,
+           enable_if_t < is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value,
+                         int > = 0 >
+inline void to_json(BasicJsonType& j, const CompatibleArrayType& bin)
+{
+    external_constructor<value_t::binary>::construct(j, typename BasicJsonType::binary_t(bin));
+}
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, const std::valarray<T>& arr)

tests/src/unit-conversions.cpp

@@ -1782,6 +1782,21 @@ TEST_CASE("std::optional")
                              "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
         CHECK_THROWS_WITH_AS(std::optional<int>(j_null),
                              "[json.exception.type_error.302] type must be number, but is null", json::type_error&);
+
+        // Assignment goes through the same overload resolution as direct
+        // construction, so it throws for the same reason. This relies on
+        // basic_json's implicit conversion operator, so it only applies
+        // when JSON_USE_IMPLICIT_CONVERSIONS is enabled (the default).
+#if JSON_USE_IMPLICIT_CONVERSIONS
+        std::optional<std::string> opt_assign;
+        CHECK_THROWS_WITH_AS(opt_assign = j_null,
+                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
+#endif
+
+        // get_to() is the correct way to obtain std::nullopt from a JSON null.
+        std::optional<std::string> opt_get_to = "placeholder";
+        j_null.get_to(opt_get_to);
+        CHECK(opt_get_to == std::nullopt);
     }
 
     SECTION("string")

tests/src/unit-regression2.cpp

@@ -1136,6 +1136,40 @@ TEST_CASE("regression tests 2")
         CHECK((decoded == json_4804::array()));
     }
 
+    SECTION("discussion #4209 - custom BinaryType direct assignment and round-tripping")
+    {
+        // Test that assigning a custom BinaryType directly creates a binary value, not an array
+        const std::vector<std::byte> original{std::byte{1}, std::byte{2}, std::byte{3}};
+        const json_4804 j = original;
+        CHECK(j.is_binary());
+        CHECK(!j.is_array());
+
+        // Test round-tripping: extracting the binary value back as the custom container type
+        const auto extracted = j.get<std::vector<std::byte>>();
+        CHECK(extracted == original);
+
+        // Test that the default json alias behavior is unchanged: std::vector<uint8_t> -> array
+        const json default_json = std::vector<std::uint8_t> {1, 2, 3};
+        CHECK(default_json.is_array());
+        CHECK(!default_json.is_binary());
+    }
+
+    SECTION("discussion #4209 - custom BinaryType extraction from parsed array")
+    {
+        // Test that extracting a custom BinaryType from a parsed JSON array still works
+        // (not just from a binary-typed node)
+        const auto j = json_4804::parse("[1,2,3]");
+        CHECK(j.is_array());
+        CHECK(!j.is_binary());
+
+        // Extracting as custom BinaryType should work from arrays
+        const auto extracted = j.get<std::vector<std::byte>>();
+        CHECK(extracted.size() == 3);
+        CHECK(extracted[0] == std::byte{1});
+        CHECK(extracted[1] == std::byte{2});
+        CHECK(extracted[2] == std::byte{3});
+    }
+
     SECTION("issue #5046 - implicit conversion of return json to std::optional no longer implicit")
     {
         const json jval{};
@@ -1165,6 +1199,46 @@ TEST_CASE("regression tests 2")
     }
 #endif
 
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+    SECTION("issue #4916 - constructing array from C++20 ranges view does not work")
+    {
+        std::vector<int> nums{1, 2, 37, 42, 21};
+        auto filteredNums = nums | std::views::filter([](int i)
+        {
+            return i > 10;
+        });
+        json const j(filteredNums);
+        CHECK(j.type() == json::value_t::array);
+        CHECK(j == json({37, 42, 21}));
+    }
+#endif
+
+    // owning_view is not available in libstdc++ < 12
+#if JSON_HAS_RANGES && !defined(__MINGW32__) && !(defined(__GLIBCXX__) && _GLIBCXX_RELEASE < 12)
+    SECTION("issue #4916 - constructing array from prvalue C++20 ranges view (owning_view)")
+    {
+        json const j(std::vector<int> {1, 2, 37, 42, 21} | std::views::filter([](int i)
+        {
+            return i > 10;
+        }));
+        CHECK(j.type() == json::value_t::array);
+        CHECK(j == json({37, 42, 21}));
+    }
+#endif
+
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+    SECTION("issue #4916 - constructing array from C++20 transform view (prvalue elements)")
+    {
+        std::vector<int> nums{1, 2, 3};
+        auto t = nums | std::views::transform([](int i) noexcept
+        {
+            return i * 2;
+        });
+        json const j(t);
+        CHECK(j.type() == json::value_t::array);
+        CHECK(j == json({2, 4, 6}));
+    }
+#endif
 }
 
 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)

tests/src/unit-user_defined_input.cpp

@@ -216,15 +216,16 @@ TEST_CASE("Parse with heterogeneous iterator and sentinel types")
 // JSON_HAS_CPP_20 (do not remove; see note at top of file)
 TEST_CASE("Parse with std::counted_iterator and std::default_sentinel_t")
 {
+    using iterator_type = std::string::const_iterator;
     const std::string json_str = R"({"key":"value","array":[1,2,3]})";
-    const auto len = static_cast<std::iter_difference_t<std::string::const_iterator>>(json_str.size());
+    const auto len = static_cast<std::iter_difference_t<iterator_type>>(json_str.size());
 
-    std::counted_iterator first(json_str.begin(), len);
+    const std::counted_iterator<iterator_type> first(json_str.begin(), len);
     const json j = json::parse(first, std::default_sentinel);
     CHECK(j["key"] == "value");
     CHECK(j["array"].size() == 3);
 
-    std::counted_iterator first2(json_str.begin(), len);
+    const std::counted_iterator<iterator_type> first2(json_str.begin(), len);
     CHECK(json::accept(first2, std::default_sentinel));
 }
 #endif