Fix crashing on invalid Concatenate usage

Author: themylogin

mypy/typeanal.py

@@ -1199,6 +1199,15 @@ def visit_callable_type(
                             "Arguments not allowed after ParamSpec.args", t, code=codes.VALID_TYPE
                         )
                     at = self.anal_type(ut, nested=nested, allow_unpack=False)
+                    if isinstance(at, Parameters):
+                        # A Parameters here comes from a misplaced Concatenate ending in
+                        # an explicit ellipsis, e.g. Callable[[Concatenate[int, ...]], None].
+                        self.fail(
+                            "Concatenate is only valid as the first argument to Callable",
+                            ut,
+                            code=codes.VALID_TYPE,
+                        )
+                        at = AnyType(TypeOfAny.from_error)
                 arg_types.append(at)
 
             if nested and arg_types:

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

@@ -2755,3 +2755,16 @@ reveal_type(Sneaky(f8, 1, y='').kwargs)  # N: Revealed type is "builtins.dict[bu
 reveal_type(Sneaky(f9, 1, y=0).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str})"
 reveal_type(Sneaky(f9, 1, y=0, z='').kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str})"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecNoCrashOnConcatenateInCallableArg]
+from typing import Callable
+from typing_extensions import Concatenate, ParamSpec
+
+P = ParamSpec("P")
+
+def takes_paramspec(f: Callable[P, object]) -> bool:
+    return False
+
+def run(callback: Callable[[Concatenate[int, ...]], None]) -> None:  # E: Concatenate is only valid as the first argument to Callable
+    takes_paramspec(callback)
+[builtins fixtures/paramspec.pyi]