TypeForm: Recognize type variables in a consistent way

Prerequisite: This issue applies after PR https://github.com/python/mypy/pull/19596 is merged.

Currently mypy sometimes allows a type variable to be assigned to a TypeForm variable and sometimes it does not. This is a problem because the behavior is inconsistent.

Specifically, mypy allows a type variable to be assigned to a TypeForm variable iff it is in a quoted type expression:

[case testTypeVarTypeFormsAreOnlyRecognizedInStringAnnotation]
...
E = TypeVar('E')
class Box(Generic[E]):
    def foo(self, e: E) -> None:
        list_of_typx: List[TypeForm] = [E]  # E: "E" is a type variable and only valid in type context
        typx1: TypeForm = E  # E: "E" is a type variable and only valid in type context
        typx2: TypeForm = 'E'

When a type variable is NOT quoted, mypy will not allow it to be assigned:

[case testEveryKindOfTypeExpressionIsAssignableToATypeFormVariable]
SomeTypeVar = TypeVar('SomeTypeVar')
typx: TypeForm
...
# NOTE: Unbound TypeVar isn't currently accepted as a TypeForm. Is that OK?
typx = SomeTypeVar  # E: Incompatible types in assignment (expression has type "TypeVar", variable has type "TypeForm[Any]")

I'd argue that TypeForm should always accept a type variable because it is a valid type expression and TypeForms are intended to accept all kinds of type expressions:

def cast[T](type_expr: TypeForm[T], value_expr: object) -> T: ...

cast(SomeTypeVar, some_expression)  # should not be an error

In summary:

• mypy should allow a type variable to be assigned to a TypeForm variable, regardless of whether it is quoted or not. Update above-referenced tests to reflect this new behavior.

Trước tiên, hãy xem xét PR prerequisite #19596, sau đó tìm các test có tên testTypeVarTypeFormsAreOnlyRecognizedInStringAnnotation và testEveryKindOfTypeExpressionIsAssignableToATypeFormVariable. Kiểm tra cách các biến kiểu có dấu ngoặc kép và không có dấu ngoặc kép được xử lý đối với các phép gán TypeForm. Được coi là hoàn tất khi cả hai trường hợp đều nhất quán cho phép các biến kiểu và các test phản ánh hành vi mới.