TypeForm: Recognize type variables in a consistent way

[davidfstr]

Prerequisite: This issue applies after PR #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.