[Bug libstdc++/104945] New: std::hash ignores the top 32 bits when size_t is 32 bit

[pdimov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104945

            Bug ID: 104945
           Summary: std::hash<std::uint64_t> ignores the top 32 bits when
                    size_t is 32 bit
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pdimov at gmail dot com
  Target Milestone: ---

A benchmark in Boost.Unordered uses uint64_t keys of the form `i << 40`:

https://github.com/boostorg/unordered/blob/33f81fd49039bccd1aa3dfd5a29ef6073b93009c/benchmark/uint64.cpp#L65

which leads to pathological behavior with std::unordered_map in 32 bit mode,
because std::hash<std::uint64_t> ignores the top 32 bits:

https://godbolt.org/z/PncKbT7aq

I'm not entirely sure whether this would be considered a bug, but decided that
it's worth reporting. Ideally, std::hash ought to take into account all bits of
the integral input, instead of truncating it to size_t.