fix(ios): build fmt pod as C++17 to fix Xcode 26.4+ consteval error#306
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Apple clang 21 (Xcode 26.4+) rejects fmt's compile-time FMT_STRING as a non-constant-expression, breaking the iOS build (fmt's format.cc). On RN 0.80.3 fmt is pinned to 11.0.2 via RCT-Folly (react/react-native#55601; fmtlib/fmt#4740). Scope the `fmt` pod to C++17 in Podfile post_install — under C++17 fmt's consteval path is disabled, sidestepping the error without patching any vendored header and staying robust across fmt versions. Only the fmt target is affected; the rest of the graph keeps C++20. The fix is upstream in React Native 0.83+, so this block can be removed once we bump to RN 0.83+.
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Problem
Apple clang 21 (Xcode 26.4+) rejects fmt's compile-time
FMT_STRING/constevalas a non-constant-expression, so the iOS build fails compiling fmt'sformat.cc:React Native vendors fmt (
RCT-Folly→fmt 11.0.2on our RN 0.80.3). CI doesn't hit it (Xcode 26.3); local builds on 26.4+/26.5 do. Tracked in facebook/react-native#55601 and fmtlib/fmt#4740.Fix
Compile only the
fmtpod as C++17 via aPodfilepost_installbuild setting. Under C++17 fmt'sconstevalpath is disabled (__cpp_constevalabsent), which sidesteps the error. Scoped totarget.name == 'fmt'so the rest of the graph keeps C++20. No vendored-header edits and robust across fmt versions.How others handle it
fmt/base.h(FMT_USE_CONSTEVAL → 0), per expo/expo#44229.fmt-as-C++17 build setting used here.All interim options disable fmt's consteval path; this is the bare-RN one.
Longevity
This is only needed because we're on RN 0.80.3. Remove this block once we bump to RN 0.83+, which carries the upstream fix (also noted in the Podfile comment).
Verify
pod install, then build the iOS example on Xcode 26.4+ — thefmttarget compiles clean.