Yes, and the way that e.g. C famously handles it is that it treats it as X being implicitly int, if it's an integral expression that produces something shorter

Yes, and the way that e.g. C famously handles it is that it treats it as X being implicitly int, if it's an integral expression that produces something shorter than long, and a double, if X is a floating-point-valued expression, and whatever happens to structs when invoking implicitly-declared functions. This, again, can be checked for the correctness at the end of the translation of a particular compilation unit, C just decided to not bother with this at all and leave it to the linker to figure out (even though it started its life as a language with 2-and-a-half passes compiler), and Pascal and its descendants decided to require either forward declarations, or using function pointers. I personally think that e.g. var F: proc(whatever): whatever; proc G() begin ... F(whatever); ... end G; proc F0(whatever): whatever begin ... G(); ... end F0; begin F := F0; end module. is a needless pessimization but e.g. Oberon does it this way.