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Xcode 26.4b1/b2/b3/RC • FMT Compile Errors (it needs upgrading) • REPRO **IS** included despite the erroneous label #55601
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Missing reproducer: We could not detect a reproducible example in your issue report. Reproducers are mandatory and we can accept only one of those as a valid reproducer:
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You can read more about about it on our website: How to report a bug.Reacted by ChaconBricks and REVI1945See https://github.com/Brett-Best/rn-fmt-reproducer which is a direct clone of the template since no changes were needed to reproduce the problem. Hopefully this pleases the bot.
- changed the title
[-]Xcode 26.4b1 • FMT Compile Errors (it needs upgrading)[/-][+]Xcode 26.4b1 • FMT Compile Errors (it needs upgrading) • REPRO **IS** included despite the erroneous label[/+]on Feb 18, 2026 Worth mentioning: I wasn't able to reproduce this after priming the build cache using Xcode 26.3.
It was reproducible after clearing build cache, or running the following:
touch ios/Pods/fmt/src/**/* ios/Pods/fmt/include/**/*
Edit: also, my project builds fine after updating
fmtto 12.1.0Reacted by WawaStill reproducible in Xcode 26.4 beta 2.
Reacted by Anirudh Mangalvedhekar, Romuald Percereau, Emmanuel Ogala, vinay-mosaic, pdymek-mohito, Sergei Metlenkin and trungledangAxonActive- changed the title
[-]Xcode 26.4b1 • FMT Compile Errors (it needs upgrading) • REPRO **IS** included despite the erroneous label[/-][+]Xcode 26.4b1/b2 • FMT Compile Errors (it needs upgrading) • REPRO **IS** included despite the erroneous label[/+]on Feb 26, 2026 was testing Xcode 26.4 beta 3, looks fixed in RN 0.84
can repro on RN 0.83
can't repro on RN 0.84/0.85
@leotm I suspect that is because of https://reactnative.dev/blog/2026/02/11/react-native-0.84#precompiled-binaries-on-ios-by-default
works for me after disabling iOS bins w
RCT_USE_PREBUILT_RNCORE=0 bundle exec pod installwhich strips- React-Core-prebuilt (0.84.1): - ReactNativeDependencies
including all transitive
- React-Core-prebuiltand updates most of the spec checksums
Reacted by Batuhan Orhan26 remaining items
RN 0.78.3 helped
# TODO: remove it later # Bump fmt version to 12.1.0 to avoid build failure with Xcode 16.4+ # Patch all react-native podspecs that pin fmt to 11.0.2 before CocoaPods resolves dependencies rn_path = File.join(__dir__, '..', 'node_modules', 'react-native') Dir.glob(File.join(rn_path, '**', '*.podspec')).each do |podspec_path| content = File.read(podspec_path) if content.include?('"fmt", "11.0.2"') || (podspec_path.end_with?('fmt.podspec') && content.include?('"11.0.2"')) content.gsub!('"fmt", "11.0.2"', '"fmt", "12.1.0"') content.gsub!(/"11\.0\.2"/, '"12.1.0"') if podspec_path.end_with?('fmt.podspec') File.write(podspec_path, content) end end- added a commit that references this issue
on Apr 6, 2026 - added a commit that references this issue
on Apr 8, 2026 - added a commit that references this issue
on Apr 8, 2026 - added a commit that references this issue
on Apr 10, 2026 rn: 0.81.4
xcode: 26.4
AI fixed with adding this on Podfile;installer.pods_project.targets.each do |target| if target.name == 'fmt' target.build_configurations.each do |config| config.build_settings['CLANG_CXX_LANGUAGE_STANDARD'] = 'c++17' end end endBut i think i need to upgrade rn.
Reacted by vitalyster and Pavel Masiuk- added a commit that references this issue
on Apr 15, 2026 - added a commit that references this issue
on Apr 21, 2026 - added 2 commits that reference this issue
on Apr 24, 2026 rn: 0.81.4 xcode: 26.4 AI fixed with adding this on Podfile;
installer.pods_project.targets.each do |target| if target.name == 'fmt' target.build_configurations.each do |config| config.build_settings['CLANG_CXX_LANGUAGE_STANDARD'] = 'c++17' end end endBut i think i need to upgrade rn.
I tried this, then build work fine :)
post_install do |installer| # Fix fmt 11.0.2 consteval errors with Xcode 26 / Clang 17+. # base.h unconditionally `#define`s FMT_USE_CONSTEVAL inside its detection # chain, so a -D flag is overwritten. Patch the header to force it to 0. fmt_base_h = "#{Pod::Config.instance.installation_root}/Pods/fmt/include/fmt/base.h" if File.exist?(fmt_base_h) text = File.read(fmt_base_h) marker = '// FMT_USE_CONSTEVAL override for Xcode 26 / Clang 17+' if text.include?(marker) puts "ℹ️ fmt/base.h already patched" else needle = "#if FMT_USE_CONSTEVAL\n# define FMT_CONSTEVAL consteval" raise "fmt/base.h structure unexpected; cannot patch FMT_USE_CONSTEVAL" unless text.include?(needle) replacement = "#{marker}\n#undef FMT_USE_CONSTEVAL\n#define FMT_USE_CONSTEVAL 0\n#{needle}" File.write(fmt_base_h, text.sub(needle, replacement)) puts "🔧 Patched fmt/base.h to force FMT_USE_CONSTEVAL=0" end else puts "ℹ️ fmt/base.h not found (skipping consteval patch)" end endReacted by M. Ahsan Rafiq, Kamlesh Manek and Beka Zandukeli- added a commit that references this issue
on Apr 28, 2026 - added a commit that references this issue
on Apr 28, 2026 - added a commit that references this issue
on May 7, 2026 - added a commit that references this issue
on May 9, 2026
Description
FMT needs upgrading so that it will compile with Xcode 26.4b1.
I modified the FMT and RCT-Folly pod specs to use
12.1.0and the compilation succeeded.I am not 100% sure but I'm suspecting this fmtlib/fmt#4065 may be the fix for the problem.
We currently depend on the legacy architecture so would be hoping this fix would qualify for an 0.81.x release.
NOTE: I did not need to fork the reproducer or use as a template because no changes were required to produce the problem.
Steps to reproduce
npm installcd ReproducerApp/iospod installDEVELOPER_DIR=/Applications/Xcode\ 26.4b1.app/Contents/Developer/ xed .React Native Version
0.83.1
Affected Platforms
Build - MacOS
Output of
npx @react-native-community/cli infoStacktrace or Logs
MANDATORY Reproducer
https://github.com/Brett-Best/rn-fmt-reproducer
Screenshots and Videos