More reliable value validness detection towards CWG1766 #398
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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`.
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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 inbit_castmakes the expression non-constant.Currently, only Clang has implemented CWG1766, while Clang's underlying
__builtin_bit_casthappens 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 whetherstatic_cast<E>(V)is a constant expression. When the answer isfalse, valueVcan't be an enumerator ofE.Rough test: https://godbolt.org/z/GbbW39MYP.