Elaboration

[dibrinsofor]

Fixes #13668, and #18166. Lays ground work for #7758

This take extends the current plugin API to support type directed refinements of function return types after mypy checks a function body, the rechecks affected call sites until a stable refined point is reached. Some messy work is done to propagate refined type information.

work could be adapted to support other annotation sites: variables, classes, parameters, etc.

1. supported:

from typing import Any

def bad(x: int) -> Any:
    return str(x)

reveal_type(bad(1))  # N: Revealed type is "builtins.str"
value: int = bad(1)  # Error! plugin developers can surface this mismatch

2. supported:

def some_method_with_union_return(some_input: str | int) -> str | int:
    return some_input


def some_other_method() -> None:
    some_method_with_union_return(5)


def another_method() -> None:
    some_method_with_union_return("foo")

unsupported:

from typing import Any

class Settings:
    retries: Any = "3"

reveal_type(Settings().retries)  # N: Revealed type is "Any"
retries: int = Settings().retries

No support to surface and refine the type of retries.

[github-actions]

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