False positive invalid-type-var from check_and_sanitize_type_parameters

[samwgoldman]

Describe the Bug

When we assign instance attributes, we need to check for unbound type variables, but the check is incorrectly also flagging type variables which are bound by the right-hand side expression.

from typing import reveal_type

def f[T](x: T) -> T:
    return x

class C:
    def m[U](self, x: U) -> U:
        return x

class D:
    def __init__(self, c: C):
        self.f = f # Unexpected invalid-type-var
        self.g = c.m # Unexpected invalid-type-var

def test(o: D):
    reveal_type(o.f) # should be [T](x: T) -> T
    reveal_type(o.g) # should be BoundMethod[C, [U](self: C, x: U) -> U]

I believe the error is happening here:

pyrefly/pyrefly/lib/alt/class/class_field.rs

Line 2323 in 6fabfc7

fn check_and_sanitize_type_parameters(

We collect the quantified types from the RHS expression, but we only want unbound quantifieds.

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN2UYANxiooAfSbEYAHXRzMMMHTABtACoBdABT5EddQEo6AWgB8BxHLo2BMBgFdK6OvjlyAxlFRw4dAMJWLraKyjSqAKo6cDBQYAA0rvoRxuZ0EUG2WXaOzq7u6F4%2BfgAimSFKdOLirBAM1doxcYke%2Bv6G5dl0TWCEygC8KtZd3bG9HIMehDQFoYzwDNq4%2BiUdwzaCImKSzDBLfcYAxN0AFrgOUJh02DB0Gjp6BqkW6ut2WxJSe7iEbEen50u11uACFzlgALL2M6YVT%2BRKRaJjNqJR4pUwWKIgeIgMiCMBQUiEBi0KAUY4ABVI%2BMJ3QwOAIdA8uHQkDYTlQDAgLMIcmOAGUYLcTgwGMQ4IgAPSSvFKQmEXhsSUwdCSzC4DxwSXM1kQdmUTnc1UqXh0VBCVDQVDYWBMllsjlcll0XDEJ3oOC89BkBhndAmESUOBGuiDGQgADMhAAjAAmcNyEAAXxxqA8XJEADFoDAKGgsHgiGRk0A

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