gh-138859: Account for ParamSpec defaults that are not lists when converting type argument list to tuple

Author: bzoracler

Lib/test/test_typing.py

@@ -761,6 +761,16 @@ class A(Generic[T, P, U]): ...
         self.assertEqual(A[float, [range]].__args__, (float, (range,), float))
         self.assertEqual(A[float, [range], int].__args__, (float, (range,), int))
 
+    def test_paramspec_and_typevar_specialization_2(self):
+        T = TypeVar("T")
+        P = ParamSpec('P', default=...)
+        U = TypeVar("U", default=float)
+        self.assertEqual(P.__default__, ...)
+        class A(Generic[T, P, U]): ...
+        self.assertEqual(A[float].__args__, (float, ..., float))
+        self.assertEqual(A[float, [range]].__args__, (float, (range,), float))
+        self.assertEqual(A[float, [range], int].__args__, (float, (range,), int))
+
     def test_typevartuple_none(self):
         U = TypeVarTuple('U')
         U_None = TypeVarTuple('U_None', default=None)

Lib/typing.py

@@ -1110,6 +1110,8 @@ def _paramspec_prepare_subst(self, alias, args):
     # Convert lists to tuples to help other libraries cache the results.
     elif isinstance(args[i], list):
         args = (*args[:i], tuple(args[i]), *args[i+1:])
+    else:
+        args = (*args[:i], args[i], *args[i + 1:])
     return args