gcc-9 rejects `std::enable_if<false,
std::integral_constant<std::integral_constant, 1> 0>::type` before it
notices that the whole expression is eliminated due to SFINAE. Use
`std::common_type` to coerce the inner integral_constant to an
appropriate integer type, which allows the expression to be well-formed
enough to reach the SFINAE check from enable_if, then be silently
removed from the overload resolution set.