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)

Beginne mit den beiden verknüpften mypy playground-Beispielen und vergleiche die Diagnosen für Vereinigungen aufrufbarer Typen, einschließlich des Falls mit einem Standardargument. Verfolge den Pfad der Argumentprüfung für Callables, der die gemeldeten Fehler erzeugt. Die Aufgabe ist abgeschlossen, wenn die Diagnosen vermitteln, dass inkompatible Callable-Signaturen nicht erfüllt werden können, während das gültige Verhalten des Standardarguments erhalten bleibt.