Fix crash on ParamSpec unification (#16251)

Fixes #16245
Fixes #16248

Unfortunately I was a bit reckless with parentheses, but in my defense
`unify_generic_callable()` is kind of broken for long time, as it can
return "solutions" like ```{1: T`1}```. We need a more principled
approach there (IIRC there is already an issue about this in the scope
of `--new-type-inference`).

(The fix is quite trivial so I am not going to wait for review too long
to save time, unless there will be some issues in `mypy_primer` etc.)

mypy/expandtype.py

@@ -236,15 +236,15 @@ def visit_param_spec(self, t: ParamSpecType) -> Type:
             return repl.copy_modified(
                 flavor=t.flavor,
                 prefix=t.prefix.copy_modified(
-                    arg_types=self.expand_types(t.prefix.arg_types + repl.prefix.arg_types),
+                    arg_types=self.expand_types(t.prefix.arg_types) + repl.prefix.arg_types,
                     arg_kinds=t.prefix.arg_kinds + repl.prefix.arg_kinds,
                     arg_names=t.prefix.arg_names + repl.prefix.arg_names,
                 ),
             )
         elif isinstance(repl, Parameters):
             assert t.flavor == ParamSpecFlavor.BARE
             return Parameters(
-                self.expand_types(t.prefix.arg_types + repl.arg_types),
+                self.expand_types(t.prefix.arg_types) + repl.arg_types,
                 t.prefix.arg_kinds + repl.arg_kinds,
                 t.prefix.arg_names + repl.arg_names,
                 variables=[*t.prefix.variables, *repl.variables],
@@ -327,12 +327,14 @@ def visit_callable_type(self, t: CallableType) -> CallableType:
             # the replacement is ignored.
             if isinstance(repl, Parameters):
                 # We need to expand both the types in the prefix and the ParamSpec itself
-                t = t.expand_param_spec(repl)
                 return t.copy_modified(
-                    arg_types=self.expand_types(t.arg_types),
+                    arg_types=self.expand_types(t.arg_types[:-2]) + repl.arg_types,
+                    arg_kinds=t.arg_kinds[:-2] + repl.arg_kinds,
+                    arg_names=t.arg_names[:-2] + repl.arg_names,
                     ret_type=t.ret_type.accept(self),
                     type_guard=(t.type_guard.accept(self) if t.type_guard is not None else None),
                     imprecise_arg_kinds=(t.imprecise_arg_kinds or repl.imprecise_arg_kinds),
+                    variables=[*repl.variables, *t.variables],
                 )
             elif isinstance(repl, ParamSpecType):
                 # We're substituting one ParamSpec for another; this can mean that the prefix

mypy/types.py

@@ -2068,16 +2068,6 @@ def param_spec(self) -> ParamSpecType | None:
         prefix = Parameters(self.arg_types[:-2], self.arg_kinds[:-2], self.arg_names[:-2])
         return arg_type.copy_modified(flavor=ParamSpecFlavor.BARE, prefix=prefix)
 
-    def expand_param_spec(self, c: Parameters) -> CallableType:
-        variables = c.variables
-        return self.copy_modified(
-            arg_types=self.arg_types[:-2] + c.arg_types,
-            arg_kinds=self.arg_kinds[:-2] + c.arg_kinds,
-            arg_names=self.arg_names[:-2] + c.arg_names,
-            is_ellipsis_args=c.is_ellipsis_args,
-            variables=[*variables, *self.variables],
-        )
-
     def with_unpacked_kwargs(self) -> NormalizedCallableType:
         if not self.unpack_kwargs:
             return cast(NormalizedCallableType, self)

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

@@ -1839,3 +1839,40 @@ g(cb, y=0, x='a')  # OK
 g(cb, y='a', x=0)  # E: Argument "y" to "g" has incompatible type "str"; expected "int" \
                    # E: Argument "x" to "g" has incompatible type "int"; expected "str"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecNoCrashOnUnificationAlias]
+import mod
+[file mod.pyi]
+from typing import Callable, Protocol, TypeVar, overload
+from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+R_co = TypeVar("R_co", covariant=True)
+Handler = Callable[P, R_co]
+
+class HandlerDecorator(Protocol):
+    def __call__(self, handler: Handler[P, R_co]) -> Handler[P, R_co]: ...
+
+@overload
+def event(event_handler: Handler[P, R_co]) -> Handler[P, R_co]: ...
+@overload
+def event(namespace: str, *args, **kwargs) -> HandlerDecorator: ...
+[builtins fixtures/paramspec.pyi]
+
+[case testParamSpecNoCrashOnUnificationCallable]
+import mod
+[file mod.pyi]
+from typing import Callable, Protocol, TypeVar, overload
+from typing_extensions import ParamSpec
+
+P = ParamSpec("P")
+R_co = TypeVar("R_co", covariant=True)
+
+class HandlerDecorator(Protocol):
+    def __call__(self, handler: Callable[P, R_co]) -> Callable[P, R_co]: ...
+
+@overload
+def event(event_handler: Callable[P, R_co]) -> Callable[P, R_co]: ...
+@overload
+def event(namespace: str, *args, **kwargs) -> HandlerDecorator: ...
+[builtins fixtures/paramspec.pyi]