Allow instantiating type[None] / types.NoneType

analyze_type_type_callee() had no case for mypy's internal NoneType,
so calling a value of type type[None] (e.g. via type(None)() or a
type[None]/type[types.NoneType] annotated callee) incorrectly
produced a "Cannot instantiate type" error, even though this is valid
at runtime and returns None.

Fixes #19660

Author: mmaxjr

mypy/checkexpr.py

@@ -1940,6 +1940,11 @@ def analyze_type_type_callee(self, item: ProperType, context: Context) -> Type:
         """
         if isinstance(item, AnyType):
             return AnyType(TypeOfAny.from_another_any, source_any=item)
+        if isinstance(item, NoneType):
+            # `type(None)` / `type[None]` is instantiable at runtime and returns None.
+            return CallableType(
+                [], [], [], NoneType(), self.named_type("builtins.function"), from_type_type=True
+            )
         if isinstance(item, Instance):
             res = type_object_type(item.type)
             if isinstance(res, CallableType):

test-data/unit/check-classes.test

@@ -3913,6 +3913,22 @@ def foo(arg: Type[Tuple[int]]):
     arg()  # E: Cannot instantiate type "type[tuple[int]]"
 [builtins fixtures/tuple.pyi]
 
+[case testTypeCanInstantiateNoneType]
+from types import NoneType
+
+type(None)()
+NoneType()
+
+def f(n: type[None]) -> None:
+    n()
+
+def g(n: type[NoneType]) -> None:  # E: NoneType should not be used as a type, please use None instead
+    n()
+
+f(NoneType)
+g(NoneType)
+[builtins fixtures/tuple.pyi]
+
 [case testTypeUsingTypeCOverloadedClass]
 from foo import *
 [file foo.pyi]