Fix urllib crash; some assert tweak

Author: Ivan Levkivskyi

mypy/applytype.py

@@ -9,7 +9,6 @@
     AnyType,
     CallableType,
     Instance,
-    Parameters,
     ParamSpecType,
     PartialType,
     TupleType,
@@ -109,8 +108,13 @@ def apply_generic_arguments(
     if param_spec is not None:
         nt = id_to_type.get(param_spec.id)
         if nt is not None:
-            if isinstance(nt, Parameters):
-                callable = callable.expand_param_spec(nt)
+            # ParamSpec expansion is special-cased, so we need to always expand callable
+            # as a whole, not expanding arguments individually.
+            callable = expand_type(callable, id_to_type)
+            assert isinstance(callable, CallableType)
+            return callable.copy_modified(
+                variables=[tv for tv in tvars if tv.id not in id_to_type]
+            )
 
     # Apply arguments to argument types.
     var_arg = callable.var_arg()

mypy/types.py

@@ -1577,7 +1577,7 @@ def __init__(
         self.arg_kinds = arg_kinds
         self.arg_names = list(arg_names)
         assert len(arg_types) == len(arg_kinds) == len(arg_names)
-        assert not any(isinstance(t, (Parameters, ParamSpecType)) for t in arg_types)
+        assert not any(isinstance(t, Parameters) for t in arg_types)
         self.min_args = arg_kinds.count(ARG_POS)
         self.is_ellipsis_args = is_ellipsis_args
         self.variables = variables or []

test-data/unit/check-parameter-specification.test

@@ -1555,3 +1555,16 @@ def test(x: U) -> U: ...
 reveal_type(dec)  # N: Revealed type is "def [P, T] (f: def (*P.args, **P.kwargs) -> T`-2) -> def (*P.args, **P.kwargs) -> builtins.list[T`-2]"
 reveal_type(dec(test))  # N: Revealed type is "def [T] (x: T`2) -> builtins.list[T`2]"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecNestedApplyNoCrash]
+from typing import Callable, TypeVar
+from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+T = TypeVar("T")
+
+def apply(fn: Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> T: ...
+def test() -> None: ...
+# TODO: avoid this error, although it may be non-trivial.
+apply(apply, test)  # E: Argument 2 to "apply" has incompatible type "Callable[[], None]"; expected "Callable[P, T]"
+[builtins fixtures/paramspec.pyi]