Cannot deduce type of generic attributes within match statements

[ariebovenberg]

Bug Report

It seems like mypy is having trouble deducing the type of generic attributes in match statements. If __match_args__ contains a generic attribute, this attribute becomes Any after case matching.

To Reproduce

Run mypy on the code below

from typing import Generic, TypeVar

T = TypeVar("T")


class A(Generic[T]):
 x: T

 __match_args__ = ("x",)

 def __init__(self, x: T):
 self.x = x


a = A("foo")
reveal_type(a) # A[str] (correct)
reveal_type(a.x) # builtins.str (correct)

match a:
 case A(b):
 reveal_type(b) # Any (incorrect! Should be builtins.str)

Expected Behavior

The total output should be:

Revealed type is "mycode.A[builtins.str]"
Revealed type is "builtins.str"
Revealed type is "builtins.str"

Actual Behavior

the last line doesn't reveal builtins.str

Revealed type is "mycode.A[builtins.str]"
Revealed type is "builtins.str"
Revealed type is "Any"

Your Environment

• Mypy version used: 0.971 (also occurs on master branch)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.5
• Operating system and version: MacOS 12.5.1

What I've found so far

• If we declare A as a minimal dataclass the attribute is correctly inferred!

  @dataclass
  class A(Generic[T]):
   x: T

• If the case explicitly matches on the attribute name, it also correctly infers the type:

  match a:
   case A(x=b):
   reveal_type(b) # builtins.str (correct)

• Issue mypy unable to narrow type of tuple elements in case clause in pattern matching #12364 may be related

• PEP626 mentions generics are special, but the examples don't appear relevant to this case.

[sobolevn]

Hi, @ariebovenberg 👋

Confession: I've never used a match statement in Python.
But, I will try to take a look (with no promises)

[sobolevn]

@ariebovenberg reviews are welcome (at least of tests!) :)