Fix crash on invalid type variable with ParamSpec

mypy/typeanal.py

@@ -459,10 +459,14 @@ def pack_paramspec_args(self, an_args: Sequence[Type]) -> list[Type]:
         # TODO: should these be re-analyzed to get rid of this inconsistency?
         count = len(an_args)
         if count > 0:
-            first_arg = get_proper_type(an_args[0])
-            if not (count == 1 and isinstance(first_arg, (Parameters, ParamSpecType, AnyType))):
-                return [Parameters(an_args, [ARG_POS] * count, [None] * count)]
-        return list(an_args)
+            if count == 1 and isinstance(get_proper_type(an_args[0]), AnyType):
+                # Single Any is interpreted as ..., rather that a single argument with Ay type.
+                # I didn't find this in the PEP, but it sounds reasonable.
+                return list(an_args)
+            if any(isinstance(a, (Parameters, ParamSpecType)) for a in an_args):
+                # Nested parameter specifications are not allowed.
+                return list(an_args)
+        return [Parameters(an_args, [ARG_POS] * count, [None] * count)]
 
     def cannot_resolve_type(self, t: UnboundType) -> None:
         # TODO: Move error message generation to messages.py. We'd first

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

@@ -1741,3 +1741,16 @@ def bar(x): ...
 
 reveal_type(bar)  # N: Revealed type is "Overload(def (x: builtins.int) -> builtins.float, def (x: builtins.str) -> builtins.str)"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecDecoratorOverloadNoCrashOnInvalidTypeVar]
+from typing import Any, Callable, List
+from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+T = 1
+
+Alias = Callable[P, List[T]]  # type: ignore
+def dec(fn: Callable[P, T]) -> Alias[P, T]: ...  # type: ignore
+f: Any
+dec(f)  # No crash
+[builtins fixtures/paramspec.pyi]