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

Commencez par reproduire l’exemple annoté add(a, b) avec mypy et lisez la discussion autour de la pull request référencée 5873. Déterminez l’inférence de contraintes attendue pour des unions par rapport à des unions, puis vérifiez que l’exemple réussit sans introduire de régressions.