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Ban bogus "existential types" #7084

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ilevkivskyi opened this issue Jun 27, 2019 · 0 comments · Fixed by #14159
Closed

Ban bogus "existential types" #7084

ilevkivskyi opened this issue Jun 27, 2019 · 0 comments · Fixed by #14159
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bug mypy got something wrong priority-1-normal topic-type-variables

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@ilevkivskyi
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It is possible to get an unbound type variable in a variable type using a generic alias, for example:

from typing import TypeVar, Dict
  
T = TypeVar('T')
Alias = Dict[str, T]

x: Alias[T]
reveal_type(x)  # Revealed type is 'builtins.dict[builtins.str, T?]'

This doesn't makes sense currently and should give an error.

See also #6520 (this looks very similar but is actually a different issue specific to type aliases).
@PeterJCLaw PeterJCLaw mentioned this issue Nov 28, 2020
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@ilevkivskyi ilevkivskyi mentioned this issue Nov 21, 2022
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ilevkivskyi added a commit that referenced this issue Nov 24, 2022
@ilevkivskyi
Support ParamSpec variables in type aliases (#14159)
13bd201 
Fixes #11855 
Fixes #7084
Fixes #10445
Should fix #4987

After thinking about this for some time, it looks like the best way to
implement this is by switching type aliases from unbound to bound type
variables. Then I can essentially simply share (or copy in one small
place, to avoid cyclic imports) all the logic that currently exists for
`ParamSpec` and `Concatenate` in `expand_type()` etc.

This will also address a big piece of tech debt, and will get some
benefits (almost) for free, such as checking bounds/values for alias
type variables, and much tighter handling of unbound type variables.

Note that in this PR I change logic for emitting some errors, I try to
avoid showing multiple errors for the same location/reason. But this is
not an essential part of this PR (it is just some test cases would
otherwise fail with even more error messages), I can reconsider if there
are objections.
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bug mypy got something wrong priority-1-normal topic-type-variables
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Support ParamSpec variables in type aliases ilevkivskyi/mypy
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