Xcode 16.3 Build Error • Implicit instantiation of undefined template 'std::char_traits<unsigned char>'

[Brett-Best]

The below issue is caused because of https://developer.apple.com/documentation/xcode-release-notes/xcode-16_3-release-notes#Deprecations:

The base template for std::char_traits has been removed. If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t or a custom character type for which you specialized std::char_traits, your code will stop working. The Standard does not mandate that a base template is provided, and such a base template is bound to be incorrect for some types, which could previously cause unexpected behavior while going undetected.

/Users/brettbest/Developer/react-native/ios/Pods/fmt/include/fmt/core.h
/Users/brettbest/Developer/react-native/ios/Pods/fmt/include/fmt/core.h:378:30 Implicit instantiation of undefined template 'std::char_traits<fmt::internal::char8_type>'

in file included from /Users/brettbest/Developer/react-native/ios/Pods/fmt/src/format.cc:8:
in file included from /Users/brettbest/Developer/react-native/ios/Pods/fmt/include/fmt/format-inl.h:19:
in file included from /Users/brettbest/Developer/react-native/ios/Pods/fmt/include/fmt/format.h:44:
In instantiation of member function 'fmt::basic_string_view<fmt::internal::char8_type>::basic_string_view' requested here
Template is declared here

[vitaut]

This looks unrelated to {fmt}, please open an issue in folly.

[Brett-Best]

@vitaut my apologies I copied across the wrong error trace, would you be able to take another look please?

EDIT: I've noticed that our FMT is horribly out of date...
I will come back to this after upgrading to the latest which on CocoaPods seems fairly old.

EDIT 2: in current master:

fmt/include/fmt/format.h

Line 960 in 7f76955

return {s, N - (std::char_traits<Char>::to_int_type(s[N - 1]) == 0 ? 1 : 0)};

I think this will be problematic but it might be surrounded my some if defs etc that mean it won't compile for newer versions of tooling 🙏🏻 that I'm not seeing