Variance inference gets lost with union types

[eekcoopuw]

Bug Report

Variance inference works out whether a type variable should be co-, contra- or in-variant, based on the way it is used as an argument type, a result type, or both.

But it seems it cannot see through a Union type used in a result. In this example, it correctly determines that T in class Example is used as both argument and result, therefore is invariant, and does not complain becuase the TypeVar is (by default) invariant. But in class Example2, T is still used as both argument and result, albeit through a Union. It seems that mypy cannot see through the union, and reports errors just as though the method had no variables in its return type. For comparison, in Example3, the type variables are not used at all in the return type, so it correctly infers that T should be contravariant; mypy gives the same error messages as Example2, suggesting that it doesn't see S|T as containing any type variables at all.

To Reproduce

from typing import Generic, Protocol, TypeVar

S = TypeVar("S")
T = TypeVar("T")

class Example(Protocol, Generic[T]):
    def method(self, arg: T) -> T:
        return arg

class Example2(Protocol, Generic[S, T]):
    def method(self, arg: T) -> S|T:        # <-- bug here
        return arg
        
class Example3(Protocol, Generic[S, T]):
    def method(self, arg: T) -> int:
        return 7

Expected Behavior

In the class Example2, I expect mypy to recognize that T is used positively in method arguments and result types, and therefore to infer that T should be an invariant variable. (The inference for S is not important for this report; but the bug does not repro if the other type is a concrete type like int.)

Actual Behavior

$ mypy example.py
example.py:10: error: Invariant type variable "S" used in protocol where covariant one is expected  [misc]
example.py:10: error: Invariant type variable "T" used in protocol where contravariant one is expected  [misc]
example.py:14: error: Invariant type variable "S" used in protocol where covariant one is expected  [misc]
example.py:14: error: Invariant type variable "T" used in protocol where contravariant one is expected  [misc]

Your Environment

• Mypy version used: 1.11.2 (compiled: yes)
• Mypy command-line flags: none, just the python file name
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.14