[ty] Add diagnostic hint for invalid assignments involving invariant generics

[sharkdp]

Summary

Relates to this FAQ entry. Consider

def modify(xs: list[int]):
    xs.append(42)

answers: list[bool] = [True, False]
modify(answers)

With this change, we now emit some info hints in the invalid-assignment diagnostic:

Test Plan

New snapshot tests

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 85.29%. The percentage of expected errors that received a diagnostic held steady at 78.13%. The number of fully passing files held steady at 64/132.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[sharkdp]

I guess we could also invest a bit more effort and refer to the name of the type parameter (_VT@dict in this case), if that seems helpful.

[sharkdp]

Here, we test that we do not emit the "Consider using … collections.abc.Mapping" hint, because Mapping is still invariant in its key type, so that wouldn't help here.

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)

[sharkdp]

There is no covariant supertype suggestion for Counter because it's type parameter corresponds to a key-like parameter of dict.

[sharkdp]

I wasn't sure about these. It seems a bit silly. If you explicitly annotated something with MutableSequence, you probably meant it?

[sharkdp]

I think it's fine to refer to that FAQ entry even for user types.

[sharkdp]

Note that there is no hint at all here. Something is more fundamentally wrong with your code if you're trying to assign a list[str] to a list[int]. And it's unlikely that we can help you by teaching you about invariant generics.