Error when using Union as constraint for TypeVar

[cdce8p]

Environment data

• Language Server version: 2021.6.0
• OS and version: darwin x64
• Python version: 3.10.0b2
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: basic

Code Snippet

from typing import TypeVar, Union

class A: ...
class B: ...
class C: ...
class D: ...

T = TypeVar("T", A, B, Union[C, D])


def do_something(value: T) -> T:
    ...

def func(value: Union[C, D]):
    value = do_something(value)  # error
    reveal_type(value)

Expected behaviour

No error. I'm not completely sure that's the way to do it, but it seems to work with mypy

test.py:16: note: Revealed type is "Union[test.C, test.D]"

Actual behaviour

Expression of type "T@do_something" cannot be assigned to declared type "C | D"
  Type "T@do_something" cannot be assigned to type "C | D"
    Type "A* | B* | C* | D*" cannot be assigned to type "C"
    Type "A* | B* | C* | D*" cannot be assigned to type "D"
    Type "A* | B* | C* | D*" cannot be assigned to type "C | D"
      "A*" is incompatible with "C"
      "B*" is incompatible with "C"
      "D*" is incompatible with "C"
      "A*" is incompatible with "D"
  ...

Argument of type "C | D" cannot be assigned to parameter "value" of type "T@do_something" in function "do_something"
  Type "C | D" is incompatible with constrained type variable "T"

Logs

--

[erictraut]

Thanks for the bug report. Yes, the type checker logic wasn't properly dealing with the case where a constrained TypeVar included both union and non-union constraints. This is now working, and the fix will be included in the next release.

[cdce8p]

Thanks for the quick fix 🚀

[jakebailey]

This issue has been fixed in version 2021.6.1, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202161-9-june-2021