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)

リンクされた2つの mypy playground の例から始め、デフォルト引数のケースを含む、呼び出し可能な型のユニオンに対する診断を比較します。報告されたエラーを生成する Callable の引数チェック経路を追跡します。互換性のない Callable シグネチャは満たせないことを診断が伝えつつ、有効なデフォルト引数の動作が維持されれば完了です。