Fix TypeForm inference in union parameters

mypy/checkexpr.py

@@ -6212,7 +6212,13 @@ def accept(
                     column=node_as_type.column,
                     is_type_form=True,
                 )
-                if is_subtype(typ1, p_type_context):
+                # Type variables in the TypeForm branch can prevent the subtype
+                # check from recognizing a type expression. Erase them only for
+                # this check; normal argument checking and inference still use
+                # the original context.
+                if is_subtype(typ1, p_type_context) or is_subtype(
+                    typ1, erasetype.erase_typevars(p_type_context)
+                ):
                     typ = typ1  # r-value type, when interpreted as a type expression
                 else:
                     typ2 = node.accept(self)

test-data/unit/check-typeform.test

@@ -255,6 +255,15 @@ typx2: TypeForm = no_such_module.NoSuchType  # E: Name "no_such_module" is not d
 
 -- Type Expression Context: Union[TypeForm, <non-TypeForm>]
 
+[case testCanPassTypeExpressionToTypeFormInUnionParameter]
+from typing import TypeVar
+from typing_extensions import Annotated, TypeForm
+T = TypeVar('T')
+def f(typx: TypeForm[T] | None) -> T: ...
+reveal_type(f(Annotated[int, 'meta']))  # N: Revealed type is "builtins.int"
+[builtins fixtures/primitives.pyi]
+[typing fixtures/typing-full.pyi]
+
 [case testAcceptsTypeFormLiteralAssignedToUnionOfTypeFormAndNonStr]
 from typing_extensions import TypeForm
 typx_or_int1: TypeForm[int | None] | int = int | None  # No error; interpret as TypeForm