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FMT_STRING consteval error with Apple clang 21 (Xcode 26.4) #4740
Description
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-nativevia 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 clangPrior 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.
Activity
vitaut commentedon Apr 10, 2026
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 commentedon Apr 10, 2026
This is incorrect. User-provided
FMT_USE_CONSTEVALis checked first and takes precedence over any auto-detection:fmt/include/fmt/base.h
Lines 117 to 118 in 9396f77
The root cause is also not quite correct because
FMT_STRING("...")is a constant expression.vitaut commentedon Apr 10, 2026
It is possible that the issue is specific to the older version of {fmt}. Does it reproduce on
main?turban commentedon Apr 10, 2026
Thanks for the clarification. After investigating further:
The fmt version in question is 11.0.2, bundled via React Native's
RCT-FollyCocoaPod (which pinsfmt = 11.0.2). In that version,FMT_USE_CONSTEVALis not guarded by#ifdef, so user-provided values via-Dflags are overwritten by the detection chain — which is why the preprocessor flag approach didn't work for us. The#ifdefguard 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:
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 commentedon Apr 10, 2026
No worries, I'm glad this is fixed already.
Merge branch 'mobile-sdk-11.5.1' into imbox-ios-mobile-sdk-11.5.1
chore(examples): patch fmt consteval for Xcode 26.4 in example-expo-53 (
Fix fmt consteval build error with Xcode 26 (Apple Clang 17+)
fix(bare-rn/ios): force C++17 on fmt/RCT-Folly pods to dodge consteva…
fix(ios): fmt consteval patch only on Xcode 26.4+
fix(deps): bump react-native to 0.83.9 to unblock iOS build on Xcode …
chore(expo-example): upgrade Expo SDK 54 to 55 and React Native 0.81 …
fix(ci): pin Xcode 26.2 for the iOS deploy job (#68)
fix macOS deps build with Apple Clang 21+ and tidy sanitizer flags
Chore: xcode fix for ios builds with old RN (software-mansion#9593)
Bump React Native to 0.86.0-rc.3 in example apps
build(ios): patch fmt for Xcode 26 compatibility
Fix: Xcode 26 build + Start crash, UI polish; bump to 1.9.7 (197)
fmt fix (real): patch fmt base.h to disable consteval on Apple clang
Fix fmt consteval build failure under Xcode 26
fix(ios): build fmt pod as C++17 to fix Xcode 26.4+ consteval error
fix(ios): build fmt pod as C++17 to fix Xcode 26.4+ consteval error
fix: bump macos-example to react-native-macos 0.81.8 for the Xcode 26…
Correct the fmt/Xcode 26 consteval fix: pin CLANG_CXX_LANGUAGE_STANDA…