Generic class with a single upper bound of a union type

[ritsuki1227]

Bug Report

When implementing the following generic class with a single upper bound of a union type, you get the following errors, while no errors are raised when using multiple upper-bounds.

To Reproduce

class Example1[T: (str, int)]:
    def foo(self, a: T) -> T:
        if isinstance(a, int):
            return a + 1
        elif isinstance(a, str):
            return a + "hello"


class Example2[T: int | str]:
    def bar(self, a: T) -> T:
        if isinstance(a, int):
            return a + 1
        elif isinstance(a, str):
            return a + "hello"

Expected Behavior

Class Example2 should raise no errors.

Actual Behavior

Class Example2 raises the following error:

error: Incompatible return value type (got "int", expected "T")  [return-value]
error: Incompatible return value type (got "str", expected "T")  [return-value]

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12.3

[JelleZijlstra]

This error is correct.

Your Example1 does not use multiple bounds, but constraints, which work differently. A TypeVar with constraints can be solved only to exactly int or str, and the method must return int if an int is given, and str if a str is given. But Example2 uses a bound, and the signature says that the return type must be exactly the same as the input type, for any consistent subtype of int | str. For example, bool is a subclass of int, so Example2().bar(True) should return bool, but in your implementation it doesn't: it returns 2, an int.

[ritsuki1227]

@JelleZijlstra Thank you for your clear explanation even though it is an expected behavior. Your examples are also easy to understand.