Xcode 26.4b1 • FMT Compile Errors (it needs upgrading) • REPRO **IS** included despite the erroneous label

[Brett-Best]

Description

FMT needs upgrading so that it will compile with Xcode 26.4b1.

I modified the FMT and RCT-Folly pod specs to use 12.1.0 and 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

1. npm install
2. cd ReproducerApp/ios
3. pod install
4. DEVELOPER_DIR=/Applications/Xcode\ 26.4b1.app/Contents/Developer/ xed .
5. Select the simulator and build

React Native Version

0.83.1

Affected Platforms

Build - MacOS

Output of npx @react-native-community/cli info

brettbest@Bretts-MacBook-Pro ~/D/r/ReproducerApp (main)> DEVELOPER_DIR=/Applications/Xcode\ 26.4b1.app/Contents/Developer/ npx @react-native-community/cli info
info Fetching system and libraries information...
System:
  OS: macOS 26.4
  CPU: (12) arm64 Apple M2 Pro
  Memory: 144.23 MB / 16.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 24.13.0
    path: /Users/brettbest/.local/share/mise/installs/node/24.13.0/bin/node
  Yarn:
    version: 1.22.22
    path: /opt/homebrew/bin/yarn
  npm:
    version: 11.6.2
    path: /Users/brettbest/.local/share/mise/installs/node/24.13.0/bin/npm
  Watchman:
    version: 2026.01.12.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.4
      - iOS 26.4
      - macOS 26.4
      - tvOS 26.4
      - visionOS 26.4
      - watchOS 26.4
  Android SDK: Not Found
IDEs:
  Android Studio: Not Found
  Xcode:
    version: 26.4/17E5159k
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 25.0.2
    path: /opt/homebrew/opt/openjdk/bin/javac
  Ruby:
    version: 3.3.0
    path: /Users/brettbest/.local/share/mise/installs/ruby/3.3.0/bin/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.0.0
    wanted: 20.0.0
  react:
    installed: 19.2.0
    wanted: 19.2.0
  react-native:
    installed: 0.83.1
    wanted: 0.83.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: true

info React Native v0.84.0 is now available (your project is running on v0.83.1).
info Changelog: https://github.com/facebook/react-native/releases/tag/v0.84.0
info Diff: https://react-native-community.github.io/upgrade-helper/?from=0.83.1&to=0.84.0
info For more info, check out "https://reactnative.dev/docs/upgrading?os=macos".

Stacktrace or Logs

/Users/brettbest/Developer/reproducer-react-native/ReproducerApp/ios/Pods/fmt/include/fmt/format-inl.h:59:24 Call to consteval function 'fmt::basic_format_string<char, fmt::basic_string_view<char> &, const char (&)[3]>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

/Users/brettbest/Developer/reproducer-react-native/ReproducerApp/ios/Pods/fmt/include/fmt/format-inl.h:60:22 Call to consteval function 'fmt::basic_format_string<char, const char (&)[7], int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

/Users/brettbest/Developer/reproducer-react-native/ReproducerApp/ios/Pods/fmt/include/fmt/format-inl.h:1387:35 Call to consteval function 'fmt::basic_format_string<char, unsigned int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

/Users/brettbest/Developer/reproducer-react-native/ReproducerApp/ios/Pods/fmt/include/fmt/format-inl.h:1391:33 Call to consteval function 'fmt::basic_format_string<char, unsigned int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

/Users/brettbest/Developer/reproducer-react-native/ReproducerApp/ios/Pods/fmt/include/fmt/format-inl.h:1394:33 Call to consteval function 'fmt::basic_format_string<char, int>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

MANDATORY Reproducer

https://github.com/Brett-Best/rn-fmt-reproducer

Screenshots and Videos

[react-native-bot]

Warning

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:

• For majority of bugs: send us a Pull Request with the RNTesterPlayground.js edited to reproduce your bug.
• If your bug is UI related: a Snack
• If your bug is build/upgrade related: a project using our Reproducer Template

You can read more about about it on our website: How to report a bug.

[Brett-Best]

See 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.

[bennettp123]

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 fmt to 12.1.0