Mypy is still too eager about outer context

Consider this example:

T = TypeVar('T')
S = TypeVar('S')

def add(x: List[T], y: List[S]) -> List[Union[T, S]]: ...

a: List[int]
b: List[str]

e: List[Union[int, str]] = add(a, b)

It fails on the last line with the following (I skip invariance notes):

error: Argument 1 to "add" has incompatible type "List[int]"; expected "List[Union[int, str]]"
error: Argument 2 to "add" has incompatible type "List[str]"; expected "List[Union[int, str]]"

If I remove the annotation for e, everything works fine.

When previously working on the external context issues, I found that I don't like how constrains for unions against unions are inferred (not that they are wrong, just counterintuitive). For example, Union[T, S] <: Union[str, int] infers T < : Union[str, int] and S <: Union[str, int], which is technically fine, but leads to errors like above.

This blocks https://github.com/python/mypy/pull/5873

Beginne damit, das annotierte add(a, b)-Beispiel mit mypy zu reproduzieren, und lies die Diskussion rund um den referenzierten Pull Request 5873. Bestimme die beabsichtigte Constraint-Inferenz für Unions gegenüber Unions und verifiziere anschließend, dass das Beispiel erfolgreich ist, ohne Regressionen einzuführen.