Fix checking of match sequence pattern against bounded type variables by brianschubert · Pull Request #18091 · python/mypy (original) (raw)

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Fixes #18089

Adds handling for bounded type variables when checking match sequence patterns.

Previously, this crashed. Now, this correctly narrows the upper bound of the type variable:

from typing import TypeVar, Sequence

T = TypeVar("T", bound=Sequence[int | str])

def accept_seq_int(x: Sequence[int]): pass

def f(x: T) -> None: match x: case [1, 2]: accept_seq_int(x) # ok: upper bound narrowed to Sequence[int] case _: accept_seq_int(x) # E: Argument 1 to "accept_seq_int" has incompatible type "T"; expected "Sequence[int]"

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

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Thanks! Would it make sense to move logic into construct_sequence_child?

Hmm, it probably would, though I'm hesitant to touch construct_sequence_child directly since it looks like it might need some more careful refactoring in the future to fix this TODO in its docstring:

Changing less now might make that a bit easier.

Changing less now might make that a bit easier.

Ok, let's merge this as is.