Unsoundly accepts generic function with generic mutable default parameter

[andersk]

Mypy unsoundly accepts a generic function with a generic mutable default parameter value. This value is evaluated once at runtime, but can be referred to with multiple incompatible types, breaking type safety.

from typing import TypeVar

T = TypeVar("T")

def f(x: T, y: list[T] = []) -> list[T]:
    return y

y_strs: list[str] = f("a")
y_ints: list[int] = f(4)
# Uh oh, y_strs and y_ints refer to the same list with incompatible types
y_strs.append("b")
print(y_ints[0] + 6)  # oops

mypy Playground link

(This is a sort of converse to #3737, where the complaint is that mypy fails to accept a generic function with a non-generic default parameter value. It further supports my claim that default parameter values should not be generalized to the generic type of the parameter at function definition time.)

Related:

• generic mutable class attributes introduce a vent in the type system #11538
• Now ClassVar cannot contain type variables, refs #11538 #11585
• Unable to specify a default value for a generic parameter #3737

[AlexWaygood]

This is a very similar issue to #5703.