xcode-26.4-fmt-consteval-workaround.md

Xcode 26.4 + fmt Build Failure Workaround (React Native / iOS)

Problem

After updating to Xcode 26.4 (including betas/RC), iOS builds can fail in Pods with errors related to the fmt C++ library.

Typical symptom:

• build fails in fmt/include/fmt/base.h
• errors reference consteval or macros around FMT_USE_CONSTEVAL

Root cause

fmt checks compiler support and enables:

FMT_USE_CONSTEVAL 1

On Xcode 26.4 toolchains, __cpp_consteval can be detected, but compilation may still fail for this path.

Workaround

Patch fmt/include/fmt/base.h after pod install and force:

• FMT_USE_CONSTEVAL 1 -> FMT_USE_CONSTEVAL 0

This is safe for app behavior; it only disables that compile-time optimization path and falls back to non-consteval handling.

Podfile patch

Add this in your Podfile (inside your target where post_install exists):

def apply_fmt_xcode_26_workaround(installer)
  fmt_base = File.join(installer.sandbox.root, 'fmt', 'include', 'fmt', 'base.h')
  return unless File.exist?(fmt_base)

  content = File.read(fmt_base)
  return if content.include?('Xcode 26 workaround: disable consteval')

  patched = content.gsub(
    /^(#elif defined\(__cpp_consteval\)\n#  define FMT_USE_CONSTEVAL) 1$/,
    "// Xcode 26 workaround: disable consteval\n\\1 0"
  )

  return if patched == content

  File.chmod(0o644, fmt_base)
  File.write(fmt_base, patched)
  Pod::UI.puts('Applied Xcode 26 fmt workaround (FMT_USE_CONSTEVAL=0)'.yellow)
end

post_install do |installer|
  react_native_post_install(
    installer,
    config[:reactNativePath],
    :mac_catalyst_enabled => false,
  )

  apply_fmt_xcode_26_workaround(installer)
end

Apply it

From your iOS folder:

pod install

Then rebuild in Xcode.

Notes

• This is a temporary workaround until upstream toolchain/library compatibility is fully resolved.
• Keep this patch only while affected by Xcode 26.4 toolchains.
• Because it runs in post_install, it is re-applied automatically on future pod install runs.