DirectXShaderCompiler vendored in SDL_shadercross fails to build with Xcode 26.4 / Apple clang 21 on macOS

[tsukinoko-kun]

Hi,
I am hitting a build failure in the vendored external/DirectXShaderCompiler when building SDL_shadercross on macOS with the current Xcode. I have replaced the with the current head from microsoft/DirectXShaderCompiler but that did not fix the issue.

Environment:

• macOS on Apple Silicon
• Xcode 26.4
• Apple clang 21.0.0
• SDK: MacOSX26.4.sdk
• SDL_shadercross 3a31448
• DirectXShaderCompiler 67a8e24

I have SDL_shadercross vendored in my game engine. The build fails while compiling vendor/SDL_shadercross/external/DirectXShaderCompiler/... sources, with errors coming from include/llvm/ADT/StringRef.h.
Representative error:

/Users/frank/Projects/bms/adventurer/vendor/SDL_shadercross/external/DirectXShaderCompiler/include/llvm/ADT/StringRef.h:579:10: error: 'is_nothrow_constructible' cannot be specialized: Users are not allowed to specialize this standard library entity [-Winvalid-specialization]
  579 |   struct is_nothrow_constructible <std::string, llvm::StringRef>
      |          ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.4.sdk/usr/include/c++/v1/__type_traits/is_nothrow_constructible.h:23:8: note: marked '_Clang::no_specializations' here
   23 | struct _LIBCPP_NO_SPECIALIZATIONS is_nothrow_constructible
      | 

I see the same error for:

• is_nothrow_constructible<std::string, llvm::StringRef>
• is_nothrow_constructible<std::string, llvm::StringRef &>
• is_nothrow_constructible<std::string, const llvm::StringRef &>
  This appears to be caused by newer libc++ in Xcode 26.4 rejecting specialization of this standard library trait.

Full log: https://pastebin.com/XKX67V5N