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msullivan opened this issue Oct 3, 2018 · 1 comment · Fixed by #19183
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Class typevars lose information from isinstance checks #5720

msullivan opened this issue Oct 3, 2018 · 1 comment · Fixed by #19183
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bug mypy got something wrong false-positive mypy gave an error on correct code priority-1-normal

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@msullivan
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from typing import Generic, TypeVar

T = TypeVar('T')

class C(Generic[T]):
    def foo(self, x: T) -> None:
        if isinstance(x, int):
            self.bar(x)

    def bar(self, x: T) -> None:
        pass

fails with Argument 1 to "bar" of "C" has incompatible type "int"; expected "T", even though x is obviously a T (that was its annotated type!)

Arguably this is just the Dreaded Intersection Bug (#3603) in a different manifestation?
@msullivan msullivan added bug mypy got something wrong false-positive mypy gave an error on correct code labels Oct 3, 2018
@ilevkivskyi
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Yes, and this is probably a hardest aspect of the problem, because I don't see an easy way to work around this using some fake TypeInfos for intersections.

@NeilGirdhar NeilGirdhar mentioned this issue Sep 7, 2020
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@NeilGirdhar NeilGirdhar mentioned this issue Nov 14, 2020
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@NeilGirdhar NeilGirdhar mentioned this issue Jan 5, 2022
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@ilevkivskyi ilevkivskyi mentioned this issue May 31, 2025
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ilevkivskyi added a commit that referenced this issue Jun 3, 2025
@ilevkivskyi
Narrow type variable bounds in binder (#19183)
5a0fa55 
Fixes #5720
Fixes #8556
Fixes #9778
Fixes #10003
Fixes #10817
Fixes #11163
Fixes #11664
Fixes #12882
Fixes #13426
Fixes #13462
Fixes #14941
Fixes #15151
Fixes #19166

This handles a (surprisingly) common edge case. The charges in
`bind_self()` and `bind_self_fast()` are tricky. I got a few "Redundant
cast" errors there, which seemed good, but then I realized that
attribute access etc. on a type variable go through slow `PyObject`
paths, so I am actually forcing `CallableType` instead of
`F(bound=CallableType)` there, since these are performance-critical
functions.
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Narrow type variable bounds in binder ilevkivskyi/mypy
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@msullivan @ilevkivskyi