Fix crash on invalid type variable with ParamSpec (#15953)

Fixes #15948

The fix is straightforward: invalid type variable resulted in applying
type arguments packing/simplification when we shouldn't. Making the
latter more strict fixes the issue.

---------

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

mypy/typeanal.py

@@ -458,11 +458,30 @@ def pack_paramspec_args(self, an_args: Sequence[Type]) -> list[Type]:
         # These do not support mypy_extensions VarArgs, etc. as they were already analyzed
         # 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 == 0:
+            return []
+        if count == 1 and isinstance(get_proper_type(an_args[0]), AnyType):
+            # Single Any is interpreted as ..., rather that a single argument with Any 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):
+            if len(an_args) > 1:
+                first_wrong = next(
+                    arg for arg in an_args if isinstance(arg, (Parameters, ParamSpecType))
+                )
+                self.fail(
+                    "Nested parameter specifications are not allowed",
+                    first_wrong,
+                    code=codes.VALID_TYPE,
+                )
+                return [AnyType(TypeOfAny.from_error)]
+            return list(an_args)
+        first = an_args[0]
+        return [
+            Parameters(
+                an_args, [ARG_POS] * count, [None] * count, line=first.line, column=first.column
+            )
+        ]
 
     def cannot_resolve_type(self, t: UnboundType) -> None:
         # TODO: Move error message generation to messages.py. We'd first
@@ -503,7 +522,11 @@ def apply_concatenate_operator(self, t: UnboundType) -> Type:
         names: list[str | None] = [None] * len(args)
 
         pre = Parameters(
-            args + pre.arg_types, [ARG_POS] * len(args) + pre.arg_kinds, names + pre.arg_names
+            args + pre.arg_types,
+            [ARG_POS] * len(args) + pre.arg_kinds,
+            names + pre.arg_names,
+            line=t.line,
+            column=t.column,
         )
         return ps.copy_modified(prefix=pre) if isinstance(ps, ParamSpecType) else pre
 
@@ -913,7 +936,7 @@ def visit_type_list(self, t: TypeList) -> Type:
             if params:
                 ts, kinds, names = params
                 # bind these types
-                return Parameters(self.anal_array(ts), kinds, names)
+                return Parameters(self.anal_array(ts), kinds, names, line=t.line, column=t.column)
             else:
                 return AnyType(TypeOfAny.from_error)
         else:

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

@@ -1741,3 +1741,26 @@ 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]
+
+[case testParamSpecErrorNestedParams]
+from typing import Generic
+from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+class C(Generic[P]): ...
+c: C[int, [int, str], str]  # E: Nested parameter specifications are not allowed
+reveal_type(c)  # N: Revealed type is "__main__.C[Any]"
+[builtins fixtures/paramspec.pyi]