Document compiler/stdlib exclusions in macro_scope.hpp (#5252)

* 📡 Document compiler/stdlib exclusions in macro_scope.hpp

Add "Known compiler/stdlib exclusions" subsections to the public documentation for
JSON_HAS_FILESYSTEM and JSON_HAS_RANGES, listing the exact compiler/stdlib versions
that are silently excluded even when feature-test macros indicate support. Each
exclusion references the originating issue. Also add a pointer note in the compiler
compatibility section linking to these details.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Code <noreply@anthropic.com>

* 🧛 fix build

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Code <noreply@anthropic.com>

docs/mkdocs/docs/api/macros/json_has_filesystem.md

@@ -19,6 +19,20 @@ The default value is detected based on the preprocessor macros `#!cpp __cpp_lib_
 `#!cpp __cpp_lib_experimental_filesystem`, `#!cpp __has_include(<filesystem>)`, or
 `#!cpp __has_include(<experimental/filesystem>)`.
 
+!!! info "Known compiler/stdlib exclusions"
+
+    Even when the feature-test macro indicates filesystem support is available, the library disables it on the following broken toolchains:
+    
+    - **MinGW + GCC 8** — disabled entirely (broken `std::filesystem` implementation; [MinGW-w64 bug 737](https://sourceforge.net/p/mingw-w64/bugs/737/))
+    - **GCC (non-Clang) < 8** — disabled (no filesystem support)
+    - **Clang < 7** — disabled (no filesystem support)
+    - **MSVC < 19.14** — disabled (no filesystem support)
+    - **iOS < 13** — disabled (no filesystem support)
+    - **macOS < Catalina (10.15)** — disabled (no filesystem support)
+    
+    If `JSON_HAS_FILESYSTEM` or `JSON_HAS_EXPERIMENTAL_FILESYSTEM` is `0` despite `__cpp_lib_filesystem` being defined, one
+    of the exclusions above likely applies to your toolchain.
+
 ## Notes
 
 - Note that older compilers or older versions of libstdc++ also require the library `stdc++fs` to be linked to for

docs/mkdocs/docs/api/macros/json_has_ranges.md

@@ -13,6 +13,18 @@ The default value is detected based on the preprocessor macro `#!cpp __cpp_lib_r
 
 When the macro is not defined, the library will define it to its default value.
 
+!!! info "Known compiler/stdlib exclusions"
+
+    Even when the feature-test macro `__cpp_lib_ranges` indicates ranges support is available, the library disables it on
+    the following incomplete or broken toolchains:
+    
+    - **GCC 11.1.0** — disabled (the shipped `<ranges>` header has a syntax error; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **libstdc++ < 11** — disabled (incomplete C++20 ranges support; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **Clang < 16 with libstdc++** — disabled (incomplete ranges support; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **libc++ < 160000** — disabled (incomplete C++20 ranges support; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    
+    If `JSON_HAS_RANGES` is `0` despite `__cpp_lib_ranges` being defined, one of the exclusions above likely applies to your toolchain.
+
 ## Examples
 
 ??? example

docs/mkdocs/docs/community/quality_assurance.md

@@ -10,6 +10,8 @@ violations will result in a failed build.
 
     Any compiler with complete C++11 support can compile the library without warnings.
 
+Note: Some modern features (like C++20 ranges or filesystem support) may be disabled on specific broken or incomplete toolchains even when standard feature-test macros indicate support. See [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md) and [`JSON_HAS_FILESYSTEM`](../api/macros/json_has_filesystem.md) for details on known exclusions.
+
 - [x] The library is compiled with 50+ different C++ compilers with different operating systems and platforms,
   including the oldest versions known to compile the library.