> If I write `def foo(x):` versus `def foo(x: int) -> float:`, one is a restricted subset of the other, but both are the language's "real" semantics.

You either are performing some wordplay here or you don't understand but type hints are not part of the semantics at all: since they are not processed at all they do not affect the behavior of the function (that's what semantics means).

EDIT: according to the language spec and current implementation

`def foo(x: int) -> float`

and

`def foo(x: float) -> int`

are the same exact function