Indexing a type also produces a GenericAlias (#17546)

Mentioned by Jelle in
#17543 (comment)

Author: hauntsaninja

mypy/checkexpr.py

@@ -4341,14 +4341,11 @@ def visit_index_with_type(
         elif isinstance(left_type, FunctionLike) and left_type.is_type_obj():
             if left_type.type_object().is_enum:
                 return self.visit_enum_index_expr(left_type.type_object(), e.index, e)
-            elif left_type.type_object().type_vars and self.chk.options.python_version >= (3, 9):
-                return self.named_type("types.GenericAlias")
-            elif (
-                left_type.type_object().fullname == "builtins.type"
-                and self.chk.options.python_version >= (3, 9)
+            elif self.chk.options.python_version >= (3, 9) and (
+                left_type.type_object().type_vars
+                or left_type.type_object().fullname == "builtins.type"
             ):
-                # builtins.type is special: it's not generic in stubs, but it supports indexing
-                return self.named_type("typing._SpecialForm")
+                return self.named_type("types.GenericAlias")
 
         if isinstance(left_type, TypeVarType) and not self.has_member(
             left_type.upper_bound, "__getitem__"

test-data/unit/pythoneval.test

@@ -1781,7 +1781,7 @@ C = str | int
 D: TypeAlias = str | int
 [out]
 _testTypeAliasNotSupportedWithNewStyleUnion.py:3: error: Invalid type alias: expression is not a valid type
-_testTypeAliasNotSupportedWithNewStyleUnion.py:3: error: Unsupported left operand type for | ("<typing special form>")
+_testTypeAliasNotSupportedWithNewStyleUnion.py:3: error: Unsupported left operand type for | ("GenericAlias")
 _testTypeAliasNotSupportedWithNewStyleUnion.py:4: error: Invalid type alias: expression is not a valid type
 _testTypeAliasNotSupportedWithNewStyleUnion.py:4: error: Unsupported left operand type for | ("Type[str]")
 _testTypeAliasNotSupportedWithNewStyleUnion.py:5: error: Invalid type alias: expression is not a valid type