Fix ParamSpec inference against TypeVarTuple (#17431)

Fixes #17278
Fixes #17127

mypy/constraints.py

@@ -1071,7 +1071,11 @@ def visit_callable_type(self, template: CallableType) -> list[Constraint]:
                 # (with literal '...').
                 if not template.is_ellipsis_args:
                     unpack_present = find_unpack_in_list(template.arg_types)
-                    if unpack_present is not None:
+                    # When both ParamSpec and TypeVarTuple are present, things become messy
+                    # quickly. For now, we only allow ParamSpec to "capture" TypeVarTuple,
+                    # but not vice versa.
+                    # TODO: infer more from prefixes when possible.
+                    if unpack_present is not None and not cactual.param_spec():
                         # We need to re-normalize args to the form they appear in tuples,
                         # for callables we always pack the suffix inside another tuple.
                         unpack = template.arg_types[unpack_present]

mypy/expandtype.py

@@ -270,6 +270,13 @@ def visit_type_var_tuple(self, t: TypeVarTupleType) -> Type:
         repl = self.variables.get(t.id, t)
         if isinstance(repl, TypeVarTupleType):
             return repl
+        elif isinstance(repl, ProperType) and isinstance(repl, (AnyType, UninhabitedType)):
+            # Some failed inference scenarios will try to set all type variables to Never.
+            # Instead of being picky and require all the callers to wrap them,
+            # do this here instead.
+            # Note: most cases when this happens are handled in expand unpack below, but
+            # in rare cases (e.g. ParamSpec containing Unpack star args) it may be skipped.
+            return t.tuple_fallback.copy_modified(args=[repl])
         raise NotImplementedError
 
     def visit_unpack_type(self, t: UnpackType) -> Type:
@@ -348,7 +355,7 @@ 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
-                return t.copy_modified(
+                expanded = t.copy_modified(
                     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,
@@ -358,6 +365,11 @@ def visit_callable_type(self, t: CallableType) -> CallableType:
                     imprecise_arg_kinds=(t.imprecise_arg_kinds or repl.imprecise_arg_kinds),
                     variables=[*repl.variables, *t.variables],
                 )
+                var_arg = expanded.var_arg()
+                if var_arg is not None and isinstance(var_arg.typ, UnpackType):
+                    # Sometimes we get new unpacks after expanding ParamSpec.
+                    expanded.normalize_trivial_unpack()
+                return expanded
             elif isinstance(repl, ParamSpecType):
                 # We're substituting one ParamSpec for another; this can mean that the prefix
                 # changes, e.g. substitute Concatenate[int, P] in place of Q.

mypy/semanal_typeargs.py

@@ -15,7 +15,7 @@
 from mypy.message_registry import INVALID_PARAM_SPEC_LOCATION, INVALID_PARAM_SPEC_LOCATION_NOTE
 from mypy.messages import format_type
 from mypy.mixedtraverser import MixedTraverserVisitor
-from mypy.nodes import ARG_STAR, Block, ClassDef, Context, FakeInfo, FuncItem, MypyFile
+from mypy.nodes import Block, ClassDef, Context, FakeInfo, FuncItem, MypyFile
 from mypy.options import Options
 from mypy.scope import Scope
 from mypy.subtypes import is_same_type, is_subtype
@@ -104,15 +104,7 @@ def visit_tuple_type(self, t: TupleType) -> None:
 
     def visit_callable_type(self, t: CallableType) -> None:
         super().visit_callable_type(t)
-        # Normalize trivial unpack in var args as *args: *tuple[X, ...] -> *args: X
-        if t.is_var_arg:
-            star_index = t.arg_kinds.index(ARG_STAR)
-            star_type = t.arg_types[star_index]
-            if isinstance(star_type, UnpackType):
-                p_type = get_proper_type(star_type.type)
-                if isinstance(p_type, Instance):
-                    assert p_type.type.fullname == "builtins.tuple"
-                    t.arg_types[star_index] = p_type.args[0]
+        t.normalize_trivial_unpack()
 
     def visit_instance(self, t: Instance) -> None:
         super().visit_instance(t)

mypy/types.py

@@ -2084,6 +2084,17 @@ 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 normalize_trivial_unpack(self) -> None:
+        # Normalize trivial unpack in var args as *args: *tuple[X, ...] -> *args: X in place.
+        if self.is_var_arg:
+            star_index = self.arg_kinds.index(ARG_STAR)
+            star_type = self.arg_types[star_index]
+            if isinstance(star_type, UnpackType):
+                p_type = get_proper_type(star_type.type)
+                if isinstance(p_type, Instance):
+                    assert p_type.type.fullname == "builtins.tuple"
+                    self.arg_types[star_index] = p_type.args[0]
+
     def with_unpacked_kwargs(self) -> NormalizedCallableType:
         if not self.unpack_kwargs:
             return cast(NormalizedCallableType, self)
@@ -2113,7 +2124,7 @@ def with_normalized_var_args(self) -> Self:
         if not isinstance(unpacked, TupleType):
             # Note that we don't normalize *args: *tuple[X, ...] -> *args: X,
             # this should be done once in semanal_typeargs.py for user-defined types,
-            # and we ourselves should never construct such type.
+            # and we ourselves rarely construct such type.
             return self
         unpack_index = find_unpack_in_list(unpacked.items)
         if unpack_index == 0 and len(unpacked.items) > 1:

test-data/unit/check-typevar-tuple.test

@@ -2407,3 +2407,56 @@ reveal_type(x)  # N: Revealed type is "__main__.C[builtins.str, builtins.int]"
 reveal_type(C(f))  # N: Revealed type is "__main__.C[builtins.str, builtins.int, builtins.int, builtins.int, builtins.int]"
 C[()]  # E: At least 1 type argument(s) expected, none given
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleAgainstParamSpecActualSuccess]
+from typing import Generic, TypeVar, TypeVarTuple, Unpack, Callable, Tuple, List
+from typing_extensions import ParamSpec
+
+R = TypeVar("R")
+P = ParamSpec("P")
+
+class CM(Generic[R]): ...
+def cm(fn: Callable[P, R]) -> Callable[P, CM[R]]: ...
+
+Ts = TypeVarTuple("Ts")
+@cm
+def test(*args: Unpack[Ts]) -> Tuple[Unpack[Ts]]: ...
+
+reveal_type(test)  # N: Revealed type is "def [Ts] (*args: Unpack[Ts`-1]) -> __main__.CM[Tuple[Unpack[Ts`-1]]]"
+reveal_type(test(1, 2, 3))  # N: Revealed type is "__main__.CM[Tuple[Literal[1]?, Literal[2]?, Literal[3]?]]"
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleAgainstParamSpecActualFailedNoCrash]
+from typing import Generic, TypeVar, TypeVarTuple, Unpack, Callable, Tuple, List
+from typing_extensions import ParamSpec
+
+R = TypeVar("R")
+P = ParamSpec("P")
+
+class CM(Generic[R]): ...
+def cm(fn: Callable[P, List[R]]) -> Callable[P, CM[R]]: ...
+
+Ts = TypeVarTuple("Ts")
+@cm  # E: Argument 1 to "cm" has incompatible type "Callable[[VarArg(Unpack[Ts])], Tuple[Unpack[Ts]]]"; expected "Callable[[VarArg(Never)], List[Never]]"
+def test(*args: Unpack[Ts]) -> Tuple[Unpack[Ts]]: ...
+
+reveal_type(test)  # N: Revealed type is "def (*args: Never) -> __main__.CM[Never]"
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleAgainstParamSpecActualPrefix]
+from typing import Generic, TypeVar, TypeVarTuple, Unpack, Callable, Tuple, List
+from typing_extensions import ParamSpec, Concatenate
+
+R = TypeVar("R")
+P = ParamSpec("P")
+T = TypeVar("T")
+
+class CM(Generic[R]): ...
+def cm(fn: Callable[Concatenate[T, P], R]) -> Callable[Concatenate[List[T], P], CM[R]]: ...
+
+Ts = TypeVarTuple("Ts")
+@cm
+def test(x: T, *args: Unpack[Ts]) -> Tuple[T, Unpack[Ts]]: ...
+
+reveal_type(test)  # N: Revealed type is "def [T, Ts] (builtins.list[T`2], *args: Unpack[Ts`-2]) -> __main__.CM[Tuple[T`2, Unpack[Ts`-2]]]"
+[builtins fixtures/tuple.pyi]