[libc++] std::type_index equality broken for anonymous namespace classes with same name with NonUnique implementation

[aeubanks]

llvm-project/libcxx/include/typeinfo

Line 117 in 6e4930c

// ========================================================================== //

describes the various ways of implementing type_info comparison.

With _LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION = 2, classes in different anonymous namespace with the same name are considered equal. #44894 already reported this, but the issue was merely worked around for Apple builds.

I believe libstdc++ handles this correctly by not doing the name comparison when it detects these sorts of anonymous namespace classes: https://github.com/gcc-mirror/gcc/blob/aedfdd340cbb4a97a8f6d432720a4df374c1e467/libstdc%2B%2B-v3/libsupc%2B%2B/typeinfo#L205.

[frederick-vs-ja]

This is essentially a duplicate of #59790 and #34255. (Note that the equality of std::type_index is based on the referenced-to std::type_info.)