Fix crash from NamedTuple placeholder

[hauntsaninja]

Fixes #17396

I'm having trouble writing a regression test, but the following reproduces the issue nicely:

rm -rf repro
mkdir repro
echo 'from mod2 import UniqueAllResult' > repro/mod1.pyi
echo '
from typing import Generic, NamedTuple, TypeVar
from mod1 import does_not_exist

_SCT = TypeVar("_SCT", bound=does_not_exist)

class UniqueAllResult(NamedTuple, Generic[_SCT]):
    values: int
' > repro/mod2.pyi

MYPYPATH=repro mypy -c 'import mod1'

[github-actions]

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

[ilevkivskyi]

This looks a bit hacky, but I guess it may be a necessary evil. Two things:

• Is there the same issue with generic TypedDicts? If yes, please fix that as well (and maybe try adding a test)
• Can you verify that a pattern like this still works? (And maybe add a corresponding test if we don't have one):

T = TypeVar("T", bound="NT")
class NT(NamedTuple, Generic[T]):
    parent: T
    item: int

[hauntsaninja]

Thanks for the review!

• Yeah, I thought about this also but wasn't able to reproduce the issue with TypedDict
• Good point, it no longer works. I opened Add regression test for NamedTuple with recursive bound #18399 for regression test, will figure out a way forward
• I also realised why I can't repro in unit tests. There's some interaction with install location (so e.g. my original repro in PR comment may not even repro for you depending on how you have mypy installed