gh-94607: Fix subclassing generics (GH-94610)

Co-authored-by: Serhiy Storchaka <3659035+serhiy-storchaka@users.noreply.github.com>

Lib/test/test_typing.py

@@ -3650,6 +3650,35 @@ def test_subclass_special_form(self):
                     class Foo(obj):
                         pass
 
+    def test_complex_subclasses(self):
+        T_co = TypeVar("T_co", covariant=True)
+
+        class Base(Generic[T_co]):
+            ...
+
+        T = TypeVar("T")
+
+        # see gh-94607: this fails in that bug
+        class Sub(Base, Generic[T]):
+            ...
+
+    def test_parameter_detection(self):
+        self.assertEqual(List[T].__parameters__, (T,))
+        self.assertEqual(List[List[T]].__parameters__, (T,))
+        class A:
+            __parameters__ = (T,)
+        # Bare classes should be skipped
+        for a in (List, list):
+            for b in (A, int, TypeVar, TypeVarTuple, ParamSpec, types.GenericAlias, types.UnionType):
+                with self.subTest(generic=a, sub=b):
+                    with self.assertRaisesRegex(TypeError, '.* is not a generic class'):
+                        a[b][str]
+        # Duck-typing anything that looks like it has __parameters__.
+        # These tests are optional and failure is okay.
+        self.assertEqual(List[A()].__parameters__, (T,))
+        # C version of GenericAlias
+        self.assertEqual(list[A()].__parameters__, (T,))
+
 class ClassVarTests(BaseTestCase):
 
     def test_basics(self):

Lib/typing.py

@@ -250,6 +250,9 @@ def _collect_parameters(args):
     """
     parameters = []
     for t in args:
+        # We don't want __parameters__ descriptor of a bare Python class.
+        if isinstance(t, type):
+            continue
         if hasattr(t, '__typing_subst__'):
             if t not in parameters:
                 parameters.append(t)

Misc/NEWS.d/next/Library/2022-07-06-16-01-08.gh-issue-94607.Q6RYfz.rst

+Fix subclassing complex generics with type variables in :mod:`typing`. Previously an error message saying ``Some type variables ... are not listed in Generic[...]`` was shown.
+:mod:`typing` no longer populates ``__parameters__`` with the ``__parameters__`` of a Python class.

Objects/genericaliasobject.c

@@ -219,6 +219,10 @@ _Py_make_parameters(PyObject *args)
     for (Py_ssize_t iarg = 0; iarg < nargs; iarg++) {
         PyObject *t = PyTuple_GET_ITEM(args, iarg);
         PyObject *subst;
+        // We don't want __parameters__ descriptor of a bare Python class.
+        if (PyType_Check(t)) {
+            continue;
+        }
         if (_PyObject_LookupAttr(t, &_Py_ID(__typing_subst__), &subst) < 0) {
             Py_DECREF(parameters);
             return NULL;