> It does not matter which is the relationship between the sizes of such types, there will always be values of the operand that cannot be represented in the result.

Hmm? Seems to me that unsigned -> larger signed works, although other conversions may not.

But yes, I generally agree that these are terrible conversions to do implicitly, given that the entire point of those types is to control the interpretation of memory at a bits-and-bytes level. Languages where implicit numeric conversions make sense are generally not languages that care so much about integer size, and the entire point of having unsigned types is to bake that range constraint in.