FMT_STRING consteval error with Apple clang 21 (Xcode 26.4)

[turban]

Compiling the fmt pod via CocoaPods on Xcode 26.4 (Apple clang 21.0.0, clang-2100.0.123.102) fails with:

error: call to consteval function 'fmt::basic_format_string<char, ...>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

Errors appear in fmt/include/fmt/format-inl.h at lines 59, 60, 1387, 1391, and 1394 — all call sites of FMT_STRING(...).

Environment

• Xcode 26.4, build version 17E192
• Apple clang 21.0.0 (clang-2100.0.123.102)
• fmt version: bundled with @maplibre/maplibre-react-native via CocoaPods

Root cause

base.h sets FMT_USE_CONSTEVAL 1 for Apple clang ≥ 14 (__apple_build_version__ >= 14000029L), enabling the consteval constructor in basic_format_string. Apple clang 21 is stricter about requiring consteval call sites to themselves be constant expressions, so the FMT_STRING(...) usages in format-inl.h — which are not constant expressions — are rejected.

Workaround

Patch base.h to treat all Apple clang versions the same as < 14 (i.e. FMT_USE_CONSTEVAL 0):

-#elif defined(__apple_build_version__) && __apple_build_version__ < 14000029L
+#elif defined(__apple_build_version__)
 #  define FMT_USE_CONSTEVAL 0  // consteval is broken in Apple clang

Prior art

PR #4703 proposed a fix for this (adding a new threshold for clang 21+) but was closed without merging and without explanation. This issue is a request to reconsider that fix or apply an alternative.

Note: passing -DFMT_CONSTEVAL= or -DFMT_USE_CONSTEVAL=0 via compiler flags does not work, because base.h unconditionally redefines both macros through its own #if/#elif chain, overwriting any -D flag.

[vitaut]

As commented on #4703 I wasn't able to repro the issue which is why it was closed, so you'll need to provide repro details. In particular do you get the error when compiling your code or {fmt} and what is the build configuration?

[vitaut]

Note: passing -DFMT_CONSTEVAL= or -DFMT_USE_CONSTEVAL=0 via compiler flags does not work, because base.h unconditionally redefines both macros through its own #if/#elif chain, overwriting any -D flag.

This is incorrect. User-provided FMT_USE_CONSTEVAL is checked first and takes precedence over any auto-detection:

fmt/include/fmt/base.h

Lines 117 to 118 in 9396f77

	#ifdef FMT_USE_CONSTEVAL
	// Use the provided definition.

The root cause is also not quite correct because FMT_STRING("...") is a constant expression.

[vitaut]

It is possible that the issue is specific to the older version of {fmt}. Does it reproduce on main?

[turban]

Thanks for the clarification. After investigating further:

The fmt version in question is 11.0.2, bundled via React Native's RCT-Folly CocoaPod (which pins fmt = 11.0.2). In that version, FMT_USE_CONSTEVAL is not guarded by #ifdef, so user-provided values via -D flags are overwritten by the detection chain — which is why the preprocessor flag approach didn't work for us. The #ifdef guard was added in 12.1.0, per the 12.1.0 release notes.

So the fix already exists in fmt 12.1.0 — the issue is that React Native hasn't upgraded yet. This is being tracked in:

• facebook/react-native#55601 — original issue report
• facebook/react-native#56225 — open PR to backport the fmt 12.1.0 bump

Apologies for the incorrect root cause analysis in the original report. Feel free to close this issue — the fix is in fmt and the ball is in React Native's court.

[vitaut]

No worries, I'm glad this is fixed already.