Summary

Consider a dict[K, V] like class with a get method:

from typing import Any, reveal_type

class Map[K, V]:
    def set(self, key: K, value: V) -> None: ...
    def get[T=None](self, key: Any, default: T = None, /) -> V | T: ...

d: Map[str, int] = Map[str, int]()

# as expected, these are both `int | None`
reveal_type(d.get("key"))  # int | None  ✅️
reveal_type(d.get("key", None))  # int | None  ✅️
# outer context should not overrule the argument type!
result: str = reveal_type(d.get("key", None))  # int | None | str ❌️

d.get("key", None) alone produces int | None as expected, but in the last case we have str as outer context. This gives an unsolvable situation for the type variable T as we would need both T <: str and T :> None. It seems ty gives priority to the outer constraint T <: str here, which seems misguided, as it produces an illogical reveal_type result: int | None | str.

But d.get("key", None) cannot produce a str!

Instead, the inner constraint T :> None should probably be prioritized in this situation.

See also mirror issues in mypy and pyrefly tracker: python/mypy#20576, facebook/pyrefly#2136

Version

No response