ban assigning function generic to instance variable declaration

```py
class C: # Note: Generic[T] missing
def bad_idea(self, x: T) -> None:
self.x = x # should be error

def nope(self, x: T) -> None:
self.x = x # E: Incompatible types in assignment (expression has type "T", variable has type "T")
```

Use the Python reproduction in the issue as the starting case and locate the type-checking path for assignments to instance variables involving function-scoped type variables. Compare the two shown methods and existing assignment diagnostics; done means the invalid assignment is rejected with the expected incompatible-types error.

Written by the indexing model from the issue text.