Errors for union of callables could be better

Given this definition (playground):

f: Callable[[int, int], None] | Callable[[int, str], None]

this code results in the following errors:

f(5, 42)  # E: Argument 2 has incompatible type "int"; expected "str"  [arg-type]
f(5, '42')  # E: Argument 2 has incompatible type "str"; expected "int"  [arg-type]

Looking at the errors (which are probably emitted in a for-loop) one would miss the big picture, which is that it's a union of two incompatible types.

Perhaps we could "meet" the callables, thus resulting in:

f(5, 42)  # E: Argument 2 has incompatible type "int"; expected <nothing>  [arg-type]
f(5, '42')  # E: Argument 2 has incompatible type "str"; expected <nothing>  [arg-type]

Notably we handle defaults well, i.e. in this code:

class FInt:
    def __call__(self, a: int, b: int=42) -> None:
        return

class FStr:
    def __call__(self, a: int, b: str='42') -> None:
        return

f: FInt | FStr

we get

f(5, 42)  # E: Argument 2 to "__call__" of "FStr" has incompatible type "int"; expected "str"  [arg-type]
f(5, '42')  # E: Argument 2 to "__call__" of "FInt" has incompatible type "str"; expected "int"  [arg-type]

but no error on

f(5)

Inizia con i due esempi collegati di mypy playground e confronta le diagnostiche per le unioni di tipi chiamabili, incluso il caso con argomento predefinito. Traccia il percorso di controllo degli argomenti dei Callable che produce gli errori segnalati. Il lavoro è completato quando le diagnostiche comunicano che non è possibile soddisfare firme di Callable incompatibili, preservando al contempo il comportamento valido dell’argomento predefinito.