Tests fail to build with libc++ 19 and above

[sin-ack]

The following code fails to build:

folly/folly/memory/test/UninitializedMemoryHacksTest.cpp

Line 310 in ac55124

testRandom<std::basic_string<signed char>>();

Because libc++ removed its deprecated std::char_traits base template as of version 19: https://releases.llvm.org/19.1.0/projects/libcxx/docs/ReleaseNotes.html#deprecations-and-removals

The base template for std::char_traits has been removed in LLVM 19. 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 currently cause unexpected behavior while going undetected.

System information

OS: Gentoo Linux amd64 (x86_64-pc-linux-gnu)
Compiler: clang version 19.1.4+libcxx
C++ stdlib implementation: libc++
Folly version: 2025.01.06.00

[elliottkember]

I have this issue in Xcode 16.3 Beta – Folly and fmt fail to build for me with this error message. I think the changes needed may be something similar to D46823611 but I don't know enough to be sure.

It may be worth re-naming this issue with something like "Does not compile in Xcode 16.3 Beta due to char_traits template" or similar, as the release of 16.3 is presumably not far off and it might be wise to prioritize a fix