Fix how type parameters are collected from parametrized Protocol bases

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Unreleased
 
+- Backport CPython PR [#137281](https://github.com/python/cpython/pull/137281),
+  fixing how type parameters are collected when a `Protocol` base class is parametrized
+  with type variables. Now, parametrized `Generic` or `Protocol` base classes always
+  dictate the number and the order of the type parameters. Patch by Brian Schubert,
+  backporting a CPython PR by Nikita Sobolev.
 - Raise `TypeError` when attempting to subclass `typing_extensions.ParamSpec` on
   Python 3.9. The `typing` implementation has always raised an error, and the
   `typing_extensions` implementation has raised an error on Python 3.10+ since

src/test_typing_extensions.py

@@ -3580,12 +3580,14 @@ class C: pass
 
     def test_defining_generic_protocols(self):
         T = TypeVar('T')
+        T2 = TypeVar('T2')
         S = TypeVar('S')
         @runtime_checkable
         class PR(Protocol[T, S]):
             def meth(self): pass
         class P(PR[int, T], Protocol[T]):
             y = 1
+        self.assertEqual(P.__parameters__, (T,))
         with self.assertRaises(TypeError):
             issubclass(PR[int, T], PR)
         with self.assertRaises(TypeError):
@@ -3594,16 +3596,23 @@ class P(PR[int, T], Protocol[T]):
             PR[int]
         with self.assertRaises(TypeError):
             P[int, str]
+        with self.assertRaisesRegex(
+            TypeError,
+            re.escape('Some type variables (~S) are not listed in Protocol[~T, ~T2]'),
+        ):
+            class ExtraTypeVars(P[S], Protocol[T, T2]): ...
         if not TYPING_3_10_0:
             with self.assertRaises(TypeError):
                 PR[int, 1]
             with self.assertRaises(TypeError):
                 PR[int, ClassVar]
         class C(PR[int, T]): pass
+        self.assertEqual(C.__parameters__, (T,))
         self.assertIsInstance(C[str](), C)
 
     def test_defining_generic_protocols_old_style(self):
         T = TypeVar('T')
+        T2 = TypeVar('T2')
         S = TypeVar('S')
         @runtime_checkable
         class PR(Protocol, Generic[T, S]):
@@ -3620,8 +3629,15 @@ class P(PR[int, str], Protocol):
                 PR[int, 1]
         class P1(Protocol, Generic[T]):
             def bar(self, x: T) -> str: ...
+        self.assertEqual(P1.__parameters__, (T,))
         class P2(Generic[T], Protocol):
             def bar(self, x: T) -> str: ...
+        self.assertEqual(P2.__parameters__, (T,))
+        msg = re.escape('Some type variables (~S) are not listed in Protocol[~T, ~T2]')
+        with self.assertRaisesRegex(TypeError, msg):
+            class ExtraTypeVars(P1[S], Protocol[T, T2]): ...
+        with self.assertRaisesRegex(TypeError, msg):
+            class ExtraTypeVars(P2[S], Protocol[T, T2]): ...
         @runtime_checkable
         class PSub(P1[str], Protocol):
             x = 1
@@ -3634,9 +3650,33 @@ def bar(self, x: str) -> str:
             with self.assertRaises(TypeError):
                 PR[int, ClassVar]
 
+    def test_protocol_parameter_order(self):
+        # https://github.com/python/cpython/issues/137191
+        T1 = TypeVar("T1")
+        T2 = TypeVar("T2", default=object)
+
+        class A(Protocol[T1]): ...
+
+        class B0(A[T2], Generic[T1, T2]): ...
+        self.assertEqual(B0.__parameters__, (T1, T2))
+
+        class B1(A[T2], Protocol, Generic[T1, T2]): ...
+        self.assertEqual(B1.__parameters__, (T1, T2))
+
+        class B2(A[T2], Protocol[T1, T2]): ...
+        self.assertEqual(B2.__parameters__, (T1, T2))
+
     if hasattr(typing, "TypeAliasType"):
         exec(textwrap.dedent(
             """
+            def test_pep695_protocol_parameter_order(self):
+                class A[T1](Protocol): ...
+                class B3[T1, T2](A[T2], Protocol):
+                    @staticmethod
+                    def get_typeparams():
+                        return (T1, T2)
+                self.assertEqual(B3.__parameters__, B3.get_typeparams())
+
             def test_pep695_generic_protocol_callable_members(self):
                 @runtime_checkable
                 class Foo[T](Protocol):

src/typing_extensions.py

@@ -3203,7 +3203,13 @@ def _is_unpacked_typevartuple(x) -> bool:
     )
 
 
-# Python 3.11+ _collect_type_vars was renamed to _collect_parameters
+# - Python 3.11+ _collect_type_vars was renamed to _collect_parameters.
+#   Breakpoint: https://github.com/python/cpython/pull/31143
+# - Python 3.13+ _collect_parameters was renamed to _collect_type_parameters.
+#   Breakpoint: https://github.com/python/cpython/pull/118900
+# - Monkey patch Generic.__init_subclass__ on <3.15 to fix type parameter
+#   collection from Protocol bases with listed parameters.
+#   Breakpoint: https://github.com/python/cpython/pull/137281
 if hasattr(typing, '_collect_type_vars'):
     def _collect_type_vars(types, typevar_types=None):
         """Collect all type variable contained in types in order of
@@ -3253,21 +3259,82 @@ def _collect_type_vars(types, typevar_types=None):
                             tvars.append(collected)
         return tuple(tvars)
 
+    def _generic_init_subclass(cls, *args, **kwargs):
+        super(Generic, cls).__init_subclass__(*args, **kwargs)
+        tvars = []
+        if '__orig_bases__' in cls.__dict__:
+            error = Generic in cls.__orig_bases__
+        else:
+            error = (Generic in cls.__bases__ and
+                     cls.__name__ != 'Protocol' and
+                     type(cls) not in (_TypedDictMeta, typing._TypedDictMeta))
+        if error:
+            raise TypeError("Cannot inherit from plain Generic")
+        if '__orig_bases__' in cls.__dict__:
+            typevar_types = (TypeVar, typing.TypeVar, ParamSpec)
+            if hasattr(typing, "ParamSpec"):  # Python 3.10+
+                typevar_types += (typing.ParamSpec,)
+            tvars = _collect_type_vars(cls.__orig_bases__, typevar_types)
+            # Look for Generic[T1, ..., Tn].
+            # If found, tvars must be a subset of it.
+            # If not found, tvars is it.
+            # Also check for and reject plain Generic,
+            # and reject multiple Generic[...].
+            gvars = None
+            basename = None
+            for base in cls.__orig_bases__:
+                if (isinstance(base, typing._GenericAlias) and
+                    base.__origin__ in (Generic, typing.Protocol, Protocol)):
+                    if gvars is not None:
+                        raise TypeError(
+                            "Cannot inherit from Generic[...] multiple times."
+                        )
+                    gvars = base.__parameters__
+                    basename = base.__origin__.__name__
+            if gvars is not None:
+                tvarset = set(tvars)
+                gvarset = set(gvars)
+                if not tvarset <= gvarset:
+                    s_vars = ', '.join(str(t) for t in tvars if t not in gvarset)
+                    s_args = ', '.join(str(g) for g in gvars)
+                    raise TypeError(
+                        f"Some type variables ({s_vars}) are"
+                        f" not listed in {basename}[{s_args}]"
+                    )
+                tvars = gvars
+        cls.__parameters__ = tuple(tvars)
+
     typing._collect_type_vars = _collect_type_vars
-else:
-    def _collect_parameters(args):
+    typing.Generic.__init_subclass__ = classmethod(_generic_init_subclass)
+elif sys.version_info < (3, 15):
+    def _collect_parameters(
+        args,
+        *,
+        enforce_default_ordering=_marker,
+        validate_all=False,
+    ):
         """Collect all type variables and parameter specifications in args
         in order of first appearance (lexicographic order).
 
+        Having an explicit `Generic` or `Protocol` base class determines
+        the exact parameter order.
+
         For example::
 
-            assert _collect_parameters((T, Callable[P, T])) == (T, P)
+            >>> P = ParamSpec('P')
+            >>> T = TypeVar('T')
+            >>> _collect_parameters((T, Callable[P, T]))
+            (~T, ~P)
+            >>> _collect_parameters((list[T], Generic[P, T]))
+            (~P, ~T)
         """
         parameters = []
 
         # A required TypeVarLike cannot appear after a TypeVarLike with default
         # if it was a direct call to `Generic[]` or `Protocol[]`
-        enforce_default_ordering = _has_generic_or_protocol_as_origin()
+        if enforce_default_ordering is _marker:
+            enforce_default_ordering = _has_generic_or_protocol_as_origin()
+
         default_encountered = False
 
         # Also, a TypeVarLike with a default cannot appear after a TypeVarTuple
@@ -3302,6 +3369,17 @@ def _collect_parameters(args):
                                             ' follows type parameter with a default')
 
                     parameters.append(t)
+            elif (
+                not validate_all
+                and isinstance(t, typing._GenericAlias)
+                and t.__origin__ in (Generic, typing.Protocol, Protocol)
+            ):
+                # If we see explicit `Generic[...]` or `Protocol[...]` base classes,
+                # we need to just copy them as-is.
+                # Unless `validate_all` is passed, in this case it means that
+                # we are doing a validation of `Generic` subclasses,
+                # then we collect all unique parameters to be able to inspect them.
+                parameters = t.__parameters__
             else:
                 if _is_unpacked_typevartuple(t):
                     type_var_tuple_encountered = True
@@ -3311,8 +3389,55 @@ def _collect_parameters(args):
 
         return tuple(parameters)
 
-    if not _PEP_696_IMPLEMENTED:
+    def _generic_init_subclass(cls, *args, **kwargs):
+        super(Generic, cls).__init_subclass__(*args, **kwargs)
+        tvars = []
+        if '__orig_bases__' in cls.__dict__:
+            error = Generic in cls.__orig_bases__
+        else:
+            error = (Generic in cls.__bases__ and
+                     cls.__name__ != 'Protocol' and
+                     type(cls) not in (_TypedDictMeta, typing._TypedDictMeta))
+        if error:
+            raise TypeError("Cannot inherit from plain Generic")
+        if '__orig_bases__' in cls.__dict__:
+            tvars = _collect_parameters(cls.__orig_bases__, validate_all=True)
+            # Look for Generic[T1, ..., Tn].
+            # If found, tvars must be a subset of it.
+            # If not found, tvars is it.
+            # Also check for and reject plain Generic,
+            # and reject multiple Generic[...].
+            gvars = None
+            basename = None
+            for base in cls.__orig_bases__:
+                if (isinstance(base, typing._GenericAlias) and
+                    base.__origin__ in (Generic, typing.Protocol, Protocol)):
+                    if gvars is not None:
+                        raise TypeError(
+                            "Cannot inherit from Generic[...] multiple times."
+                        )
+                    gvars = base.__parameters__
+                    basename = base.__origin__.__name__
+            if gvars is not None:
+                tvarset = set(tvars)
+                gvarset = set(gvars)
+                if not tvarset <= gvarset:
+                    s_vars = ', '.join(str(t) for t in tvars if t not in gvarset)
+                    s_args = ', '.join(str(g) for g in gvars)
+                    raise TypeError(
+                        f"Some type variables ({s_vars}) are"
+                        f" not listed in {basename}[{s_args}]"
+                    )
+                tvars = gvars
+        cls.__parameters__ = tuple(tvars)
+
+    if _PEP_696_IMPLEMENTED:
+        typing._collect_type_parameters = _collect_parameters
+        typing._generic_init_subclass = _generic_init_subclass
+    else:
         typing._collect_parameters = _collect_parameters
+        typing.Generic.__init_subclass__ = classmethod(_generic_init_subclass)
+
 
 # Backport typing.NamedTuple as it exists in Python 3.13.
 # In 3.11, the ability to define generic `NamedTuple`s was supported.