Summary

iOS build fails when using Expo SDK 55 (React Native 0.83.2) with Xcode 26.4 and buildReactNativeFromSource: true. The bundled fmt 11.0.2 library produces 5 compilation errors related to consteval.

This only affects projects that build React Native from source. Prebuilt binaries are not affected.

Environment

• Expo SDK: 55
• React Native: 0.83.2
• fmt: 11.0.2 (bundled via react-native/third-party-podspecs/fmt.podspec)
• Xcode: 26.4 (Build version 17E192)
• macOS: Darwin 25.3.0

Error Output

❌  (ios/Pods/fmt/include/fmt/format-inl.h:59:24)
  57 |   auto it = appender(out);
  58 |   if (message.size() <= inline_buffer_size - error_code_size)
> 59 |     fmt::format_to(it, FMT_STRING("{}{}"), message, SEP);
     |                        ^ 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

5 similar errors all in format-inl.h, all caused by FMT_STRING macro using consteval.

Root Cause

fmt 11.0.2 defines FMT_USE_CONSTEVAL=1 when it detects Clang ≥ 11.01 (see fmt/base.h line 128). However, Apple Clang shipped with Xcode 26.4 enforces stricter consteval rules than previous versions, causing FMT_STRING macro expansions to fail.

Related upstream issues:

• Compilation error with AppleClang 16.0.0.16000026 fmtlib/fmt#4181 (AppleClang 16 consteval error)
• Formatting with std::string fail to compile with consteval fmtlib/fmt#4046 (consteval compilation failure)

Reproduction

npx create-expo-app@latest fmt-test --template blank-typescript
cd fmt-test
npx expo install expo-build-properties

Then update app.json to enable building React Native from source:

{
  "expo": {
    "plugins": [
      ["expo-build-properties", {
        "ios": {
          "buildReactNativeFromSource": true
        }
      }]
    ]
  }
}

Then build:

npx expo run:ios  # on Xcode 26.4 — fails with 5 fmt consteval errors

Note: Without buildReactNativeFromSource: true, the build succeeds because prebuilt binaries are used and fmt is not compiled from source.

Workaround

Create an Expo Config Plugin that patches the Podfile to disable consteval in fmt:

// plugins/ios/withFmtFix.js
const { withDangerousMod } = require("@expo/config-plugins");
const fs = require("node:fs");
const path = require("node:path");

const withFmtFix = (config) => {
  return withDangerousMod(config, [
    "ios",
    async (config) => {
      const podfilePath = path.join(
        config.modRequest.platformProjectRoot,
        "Podfile"
      );
      if (!fs.existsSync(podfilePath)) return config;

      let content = fs.readFileSync(podfilePath, "utf-8");
      if (content.includes("FMT_USE_CONSTEVAL")) return config;

      const patchCode = `
    # Fix fmt 11.0.2 consteval compilation error with Xcode 26.4+
    fmt_base = File.join(installer.sandbox.pod_dir('fmt'), 'include', 'fmt', 'base.h')
    if File.exist?(fmt_base)
      content = File.read(fmt_base)
      patched = content.gsub(/^#\\s*define FMT_USE_CONSTEVAL 1$/, '# define FMT_USE_CONSTEVAL 0')
      if patched != content
        File.chmod(0644, fmt_base)
        File.write(fmt_base, patched)
      end
    end`;

      content = content.replace(
        /(react_native_post_install\([^)]*\)[\s\S]*?\))([\s\n]*end[\s\n]*end)/,
        `$1\n${patchCode}\n$2`
      );
      fs.writeFileSync(podfilePath, content);
      return config;
    },
  ]);
};

module.exports = withFmtFix;

Then add "./plugins/ios/withFmtFix" to the plugins array in app.config.ts.

Expected Behavior

npx expo run:ios should compile successfully on Xcode 26.4, even with buildReactNativeFromSource: true.

Suggested Fix

Either:

1. Bump fmt to a version that handles newer Apple Clang correctly, or
2. Add FMT_USE_CONSTEVAL=0 to fmt's build settings in the generated Podfile template when building from source