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`.
The comments in the code regarding MAGIC_ENUM_RANGE_MIN and MAGIC_ENUM_RANGE_MAX values were not updated according to the latest changes on those default values.
Thomas Khyn