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The poisoned_hash_helper header was relying on an implicit forward declaration of std::hash located in <type_traits>. When we improve the modularization of the library, that causes issues, in addition to being a fundamentally non-portable assumption in the test suite. It turns out that the reason for relying on a forward declaration is to be able to test that std::hash is *not* provided if we don't include any header that provides it. But testing that is actually both non-portable and not really useful. Indeed, what harm does it make if additional headers provide std::hash specializations? That would certainly be conforming -- the Standard never requires an implementation to avoid providing a declaration when a given header is included, instead it mandates what *must* be provided for sure. In that spirit, it would be conforming for e.g. `<cstddef>` to define the hash specializations if that was our desire. I also don't read https://wg21.link/P0513R0 as going against that statement. Hence, this patch just removes that test which doesn't carry its weight. Fixes #56938