This is completely off topic here, but the first promotion of Int64 -> Float64 is also iffy as not every Int64 is exactly representable. The Int64 -> Float16 promotion also looks weird to me because Float16 range is so small
You think negative numbers in a signed type are a 1-in-a-1000 case, really?
Silently casting signed integers to unsigned is a terrible idea, and a recipe for bugs. And completely unnecessary to boot, because you could promote Int8 to Int16, and (Int16, UInt16) to (Int32, Int32).
Related, an integer bug that can't be fixed in C is that "unsigned short" promotes to "int" instead of "unsigned int", so this doesn't produce the result you want.
unsigned x = (unsigned short)65535 * (unsigned short)65535;
(It overflows even though it's less than UINT_MAX, and worse it's UB.)