Generate error if mixing old-style and new-style type vars

mypy/messages.py

@@ -2421,6 +2421,10 @@ def annotation_in_unchecked_function(self, context: Context) -> None:
             code=codes.ANNOTATION_UNCHECKED,
         )
 
+    def type_parameters_should_be_declared(self, undeclared: list[str], context: Context) -> None:
+        names = ", ".join('"' + n + '"' for n in undeclared)
+        self.fail(f"All type parameters should be declared ({names} not declared)", context)
+
 
 def quote_type_string(type_string: str) -> str:
     """Quotes a type representation for use in messages."""

mypy/semanal.py

@@ -1101,6 +1101,14 @@ def update_function_type_variables(self, fun_type: CallableType, defn: FuncItem)
             fun_type.variables, has_self_type = a.bind_function_type_variables(fun_type, defn)
             if has_self_type and self.type is not None:
                 self.setup_self_type()
+            if defn.type_args:
+                bound_fullnames = {v.fullname for v in fun_type.variables}
+                declared_fullnames = {self.qualified_name(p.name) for p in defn.type_args}
+                extra = sorted(bound_fullnames - declared_fullnames)
+                if extra:
+                    self.msg.type_parameters_should_be_declared(
+                        [n.split(".")[-1] for n in extra], defn
+                    )
             return has_self_type
 
     def setup_self_type(self) -> None:
@@ -2086,11 +2094,17 @@ class Foo(Bar, Generic[T]): ...
                 self.fail("Duplicate type variables in Generic[...] or Protocol[...]", context)
             declared_tvars = remove_dups(declared_tvars)
             if not set(all_tvars).issubset(set(declared_tvars)):
-                self.fail(
-                    "If Generic[...] or Protocol[...] is present"
-                    " it should list all type variables",
-                    context,
-                )
+                if defn.type_args:
+                    undeclared = sorted(set(all_tvars) - set(declared_tvars))
+                    self.msg.type_parameters_should_be_declared(
+                        [tv[0] for tv in undeclared], context
+                    )
+                else:
+                    self.fail(
+                        "If Generic[...] or Protocol[...] is present"
+                        " it should list all type variables",
+                        context,
+                    )
                 # In case of error, Generic tvars will go first
                 declared_tvars = remove_dups(declared_tvars + all_tvars)
         else:

test-data/unit/check-python312.test

@@ -1164,7 +1164,6 @@ class C[T]:
 
 [case testPEP695InvalidGenericOrProtocolBaseClass]
 # mypy: enable-incomplete-feature=NewGenericSyntax
-
 from typing import Generic, Protocol, TypeVar
 
 S = TypeVar("S")
@@ -1181,3 +1180,24 @@ class P[T](Protocol[T]):  # E: No arguments expected for "Protocol" base class
     pass
 class P2[T](Protocol[S]):  # E: No arguments expected for "Protocol" base class
     pass
+
+[case testPEP695MixNewAndOldStyleGenerics]
+# mypy: enable-incomplete-feature=NewGenericSyntax
+from typing import TypeVar
+
+S = TypeVar("S")
+U = TypeVar("U")
+
+def f[T](x: T, y: S) -> T | S: ...  # E: All type parameters should be declared ("S" not declared)
+def g[T](x: S, y: U) -> T | S | U: ...  # E: All type parameters should be declared ("S", "U" not declared)
+
+def h[S: int](x: S) -> S:
+   a: int = x
+   return x
+
+class C[T]:
+    def m[X, S](self, x: S, y: U) -> X | S | U: ...  # E: All type parameters should be declared ("U" not declared)
+    def m2(self, x: T, y: S) -> T | S: ...
+
+class D[T](C[S]):  # E: All type parameters should be declared ("S" not declared)
+    pass