[PATCH] libstdc++: Fix compare_three_way for constexpr and Clang

[Paul Keir]

Hi,

The current compare_three_way implementation makes provision for constant evaluation contexts (avoiding reinterpret_cast etc.), but the approach fails with Clang; when it compares two const volatile void pointers: "comparison between unequal pointers to void has unspecified result". I include a fix and test.

Could someone commit the attached patch for me?

Thanks,
Paul


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