I ended up not actually using these but it seemed a shame to throw them away so here they are for copy pasterity.
underlyling_type:
template< typename T >
constexpr bool IsSigned() { return int( T( -1 ) ) == -1; }
template< size_t N, bool Signed > struct MakeIntType;
template<> struct MakeIntType< 1, true > { using T = s8; };
template<> struct MakeIntType< 2, true > { using T = s16; };
template<> struct MakeIntType< 4, true > { using T = s32; };
template<> struct MakeIntType< 8, true > { using T = s64; };
template<> struct MakeIntType< 1, false > { using T = u8; };
template<> struct MakeIntType< 2, false > { using T = u16; };
template<> struct MakeIntType< 4, false > { using T = u32; };
template<> struct MakeIntType< 8, false > { using T = u64; };
template< typename E >
using UnderlyingType = MakeIntType< sizeof( E ), IsSigned< E >() >::T;
ADDENDUM: that breaks in some cases, the __underlying_type intrinsic works on all compilers.
numeric_limits:
template< typename T > constexpr T MaxInt;
template<> constexpr u8 MaxInt< u8 > = U8_MAX;
template<> constexpr u16 MaxInt< u16 > = U16_MAX;
template<> constexpr u32 MaxInt< u32 > = U32_MAX;
template<> constexpr u64 MaxInt< u64 > = U64_MAX;