Avoid std::basic_string<CharType> for non-character output_adapter CharType

output_adapter<CharType, StringType> defaulted StringType to
std::basic_string<CharType>, and (with JSON_NO_IO undefined) always
declared a std::basic_ostream<CharType>&-taking constructor. For
CharType with no non-deprecated std::char_traits specialization (only
std::uint8_t is ever used this way, by the binary writers), simply
naming either type - as an unused default template argument, or as an
unused, never-called constructor's parameter type - instantiates
std::char_traits<CharType> merely to name it, which some standard
libraries mark deprecated: with the library-wide -Wdocumentation
pragma (item 2's other half, left for a later commit) temporarily
removed, an Apple clang 21 / libc++ TU calling json::to_cbor(j, vec)
with std::vector<std::uint8_t>& got one -Wdeprecated-declarations
warning per binary writer at the old output_adapters.hpp:193.

Replaced the eager std::basic_string<CharType> / std::basic_ostream
<CharType> defaults with a bool-tagged partial specialization (not
std::conditional, which requires naming both branches' types up
front regardless of which is selected, reproducing the same warning)
that only ever names std::basic_string<CharType> / std::basic_ostream
<CharType> when CharType is actually one of char, wchar_t, char16_t,
char32_t, or (with __cpp_lib_char8_t) char8_t. For any other
CharType, output_adapter's StringType and ostream-constructor
parameter fall back to two distinct empty placeholder types, kept
distinct so the two constructor overloads do not collide into a
single redeclaration.

Public API / behavior: passing a std::basic_string<std::uint8_t>& or
std::basic_ostream<std::uint8_t>& directly to a binary writer's
output_adapter now fails to compile instead of compiling with a
deprecation warning; this was neither documented nor tested. All
documented uses (std::vector<CharType>, std::basic_ostream<CharType>
and StringType for character CharType) are unaffected.

Verified with Apple clang 21 / libc++, with the two -Wdocumentation*
"ignored" pragma lines in macro_scope.hpp temporarily removed and
-std=c++11/c++20 plus the project's -Weverything flag set: calling
to_cbor/to_msgpack/to_ubjson/to_bjdata/to_bson/to_bon8 on a
std::vector<std::uint8_t> now produces no char_traits<unsigned char>
(or any other) deprecation warning, while the char-based string- and
ostream-adapter paths, and a to_cbor/from_cbor round trip, still
compile and run correctly; also verified with GCC 16.2.0. Ran the
full local test suite, including the binary-format unit tests
(unit-cbor, unit-msgpack, unit-ubjson, unit-bjdata, unit-bson,
unit-bon8, unit-binary_writer_sinks, unit-binary_formats,
unit-custom-binary-type): 129/129 passing.

#5725 item 2 (step a)

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

include/nlohmann/detail/output/output_adapters.hpp

@@ -13,6 +13,7 @@
 #include <iterator> // back_inserter
 #include <memory> // shared_ptr, make_shared
 #include <string> // basic_string
+#include <type_traits> // conditional, integral_constant, is_same
 #include <utility> // move
 #include <vector> // vector
 
@@ -192,7 +193,82 @@ class output_adapter_sink
     output_adapter_t<CharType> oa;
 };
 
-template<typename CharType, typename StringType = std::basic_string<CharType>>
+/// @brief whether std::basic_string<CharType> has a non-deprecated std::char_traits
+///        specialization, and is therefore usable as output_adapter's default StringType
+///
+/// std::char_traits is only guaranteed (and, on some standard libraries, only
+/// implemented without a deprecation warning) for the character types listed
+/// below; std::char_traits<T> for any other T (e.g. std::uint8_t, as used by the
+/// binary writers) is a non-standard extension some standard libraries deprecate.
+/// See https://github.com/nlohmann/json/issues/5725 item 2.
+template<typename CharType>
+struct is_output_adapter_string_char_type : std::integral_constant < bool,
+    std::is_same<CharType, char>::value ||
+    std::is_same<CharType, wchar_t>::value ||
+    std::is_same<CharType, char16_t>::value ||
+    std::is_same<CharType, char32_t>::value
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
+    || std::is_same<CharType, char8_t>::value
+#endif
+    > {};
+
+/// @brief placeholder type for output_adapter's StringType and (with JSON_NO_IO
+///        undefined) its std::basic_ostream constructor parameter, for CharType
+///        with no non-deprecated std::char_traits specialization
+///
+/// Never actually used: the StringType- and std::basic_ostream-based
+/// output_adapter constructors are neither documented nor tested for such
+/// CharType (only the std::vector-based constructor is used for them, by the
+/// binary writers). Naming std::basic_string<CharType> or
+/// std::basic_ostream<CharType> anywhere such a constructor would otherwise be
+/// declared - even as an unused default template argument or an unused,
+/// never-called overload - instantiates std::char_traits<CharType> merely to
+/// name the type, which is exactly what triggers the deprecation warning this
+/// placeholder avoids.
+template<typename CharType>
+struct output_adapter_no_string_type {};
+
+// Select output_adapter's default StringType (and, below, its ostream
+// constructor's parameter type) via partial specialization, not
+// std::conditional: std::conditional<B, T, F> requires both T and F to be named
+// as template arguments up front, which would still instantiate (and thus name)
+// std::basic_string<CharType> / std::basic_ostream<CharType> for every CharType,
+// defeating the point. A bool non-type parameter with two specializations only
+// ever names the type that is actually selected.
+template<typename CharType, bool = is_output_adapter_string_char_type<CharType>::value>
+struct output_adapter_default_string_type
+{
+    using type = output_adapter_no_string_type<CharType>;
+};
+
+template<typename CharType>
+struct output_adapter_default_string_type<CharType, true>
+{
+    using type = std::basic_string<CharType>;
+};
+
+#ifndef JSON_NO_IO
+/// distinct from output_adapter_no_string_type, so the placeholder overloads of
+/// output_adapter's constructor (used when CharType is not a character type)
+/// stay distinct overloads instead of colliding into a single redeclaration
+template<typename CharType>
+struct output_adapter_no_ostream_type {};
+
+template<typename CharType, bool = is_output_adapter_string_char_type<CharType>::value>
+struct output_adapter_ostream_type
+{
+    using type = output_adapter_no_ostream_type<CharType>;
+};
+
+template<typename CharType>
+struct output_adapter_ostream_type<CharType, true>
+{
+    using type = std::basic_ostream<CharType>;
+};
+#endif  // JSON_NO_IO
+
+template < typename CharType, typename StringType =
+           typename output_adapter_default_string_type<CharType>::type >
 class output_adapter
 {
   public:
@@ -201,7 +277,7 @@ class output_adapter
         : oa(std::make_shared<output_vector_adapter<CharType, AllocatorType>>(vec)) {}
 
 #ifndef JSON_NO_IO
-    output_adapter(std::basic_ostream<CharType>& s)
+    output_adapter(typename output_adapter_ostream_type<CharType>::type& s)
         : oa(std::make_shared<output_stream_adapter<CharType>>(s)) {}
 #endif  // JSON_NO_IO