Disable consteval on Apple clang 21

[lapfelix]

Summary

Disable the FMT_CONSTEVAL path on Apple clang 21.

Context

While building Transit's vendored copy of fmt with Xcode 26.4 beta 2 (Apple clang 21.0.0 / clang-2100.0.121.2), clang rejects several FMT_STRING(...) call sites with errors like:

error: call to consteval function 'fmt::basic_format_string<...>' is not a constant expression

The first failure I hit was in format-inl.h while compiling a downstream translation unit that includes fmt/core.h.

Change

This introduces FMT_USE_CONSTEVAL and disables it for Apple clang 21+, leaving the existing FMT_CONSTEVAL path unchanged for other toolchains.

Verification

Built the affected downstream target successfully after this change:

xcodebuild -project Submodules/transitLib/xcode_build/transitLib.xcodeproj \
  -scheme bgtfs \
  -destination 'generic/platform=iOS Simulator' \
  build

[lakshit-acv]

We can possibly give reason or some explanation as to why the PR has been closed. Helps a lot towards getting more context. Thanks !

[turban]

We're hitting the same issue in a React Native / Expo project building with Xcode 16 (Apple clang 16, not quite clang 21 but the same failure mode). The errors are identical — call to consteval function '...basic_format_string...' is not a constant expression in format-inl.h.

We worked around it by patching base.h in a CocoaPods post_install hook, removing the < 14000029L version threshold so that all Apple clang versions take the FMT_USE_CONSTEVAL 0 path:

content = content.gsub(
  'defined(__apple_build_version__) && __apple_build_version__ < 14000029L',
  'defined(__apple_build_version__)'
)

The approach in this PR (adding a new threshold for clang 21+) seems like the cleaner upstream fix. Would the maintainers consider reopening or a follow-up PR? The workaround burden is falling on every downstream project that builds fmt via CocoaPods with a recent Xcode.

[vitaut]

The error "not a constant expression" usually indicates that the format string is invalid. You'll need to provide a repro to demonstrate that there is an actual issue with consteval in clang (which is unlikely).