[pull] master from Neargye:master #18
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CWG1766 made out-of-range cast to enumeration without fixed underlying type raise undefined behavior. Such UB arguably also applies to `bit_cast`, although it's not required that UB in `bit_cast` makes the expression non-constant. Currently, only Clang has implemented CWG1766, while Clang's underlying `__builtin_bit_cast` happens to be a workaround. However, it's more reliable to me to rely on the guarantee that core language UB causes constant evaluation failure. The approach in this patch effectively detects whether `std::integral_constant<E, static_cast<E>(V)>` is a valid type, which is equivalent to whether `static_cast<E>(V)` is a constant expression. When the answer is `false`, value `V` can't be an enumerator of `E`.
) Using `static constexpr` in a header file does not seem to be correct - see [1] - and generates a bug when building the magic_enum module with GCC 15 [1] https://isocpp.org/blog/2018/05/quick-q-use-of-constexpr-in-header-file
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