Better handling of Any/object in variadic generics (#18643)

Fixes #18407
Fixes #17184
Fixes #16567

There are three things here:
* Allow erased variadic callables with non-empty prefix to be supertypes
of the non-erased ones. This relaxes a bit callable subtyping in
general, but IMO this makes sense, people who want to be strict should
simply use `*args: object` instead. An alternative would be to track
erased variadic callables explicitly, which is ugly and fragile.
* Add important missing case in `subtypes.py` for `*Ts` w.r.t.
`Any`/`object` that handles similar situations for variadic instances
and tuples (here however there is nothing special about `Any` vs
`object`).
* I also fix inconsistency in join uncovered by the above two.

The changes in `expandtype.py` are no-op, I just noticed potential
danger while playing with this, so wanted to highlight it with comments
for the future.

Author: ilevkivskyi

mypy/erasetype.py

@@ -203,6 +203,14 @@ def visit_tuple_type(self, t: TupleType) -> Type:
                     return unpacked
         return result
 
+    def visit_callable_type(self, t: CallableType) -> Type:
+        result = super().visit_callable_type(t)
+        assert isinstance(result, ProperType) and isinstance(result, CallableType)
+        # Usually this is done in semanal_typeargs.py, but erasure can create
+        # a non-normal callable from normal one.
+        result.normalize_trivial_unpack()
+        return result
+
     def visit_type_var_tuple(self, t: TypeVarTupleType) -> Type:
         if self.erase_id(t.id):
             return t.tuple_fallback.copy_modified(args=[self.replacement])

mypy/expandtype.py

@@ -226,6 +226,8 @@ def visit_instance(self, t: Instance) -> Type:
             if isinstance(arg, UnpackType):
                 unpacked = get_proper_type(arg.type)
                 if isinstance(unpacked, Instance):
+                    # TODO: this and similar asserts below may be unsafe because get_proper_type()
+                    # may be called during semantic analysis before all invalid types are removed.
                     assert unpacked.type.fullname == "builtins.tuple"
                     args = list(unpacked.args)
         return t.copy_modified(args=args)
@@ -333,10 +335,7 @@ def interpolate_args_for_unpack(self, t: CallableType, var_arg: UnpackType) -> l
 
         var_arg_type = get_proper_type(var_arg.type)
         new_unpack: Type
-        if isinstance(var_arg_type, Instance):
-            # we have something like Unpack[Tuple[Any, ...]]
-            new_unpack = UnpackType(var_arg.type.accept(self))
-        elif isinstance(var_arg_type, TupleType):
+        if isinstance(var_arg_type, TupleType):
             # We have something like Unpack[Tuple[Unpack[Ts], X1, X2]]
             expanded_tuple = var_arg_type.accept(self)
             assert isinstance(expanded_tuple, ProperType) and isinstance(expanded_tuple, TupleType)
@@ -348,6 +347,11 @@ def interpolate_args_for_unpack(self, t: CallableType, var_arg: UnpackType) -> l
             fallback = var_arg_type.tuple_fallback
             expanded_items = self.expand_unpack(var_arg)
             new_unpack = UnpackType(TupleType(expanded_items, fallback))
+        # Since get_proper_type() may be called in semanal.py before callable
+        # normalization happens, we need to also handle non-normal cases here.
+        elif isinstance(var_arg_type, Instance):
+            # we have something like Unpack[Tuple[Any, ...]]
+            new_unpack = UnpackType(var_arg.type.accept(self))
         else:
             # We have invalid type in Unpack. This can happen when expanding aliases
             # to Callable[[*Invalid], Ret]

mypy/join.py

@@ -299,6 +299,9 @@ def visit_param_spec(self, t: ParamSpecType) -> ProperType:
     def visit_type_var_tuple(self, t: TypeVarTupleType) -> ProperType:
         if self.s == t:
             return t
+        if isinstance(self.s, Instance) and is_subtype(t.upper_bound, self.s):
+            # TODO: should we do this more generally and for all TypeVarLikeTypes?
+            return self.s
         return self.default(self.s)
 
     def visit_unpack_type(self, t: UnpackType) -> UnpackType:
@@ -350,6 +353,8 @@ def visit_instance(self, t: Instance) -> ProperType:
             return join_types(t, self.s)
         elif isinstance(self.s, LiteralType):
             return join_types(t, self.s)
+        elif isinstance(self.s, TypeVarTupleType) and is_subtype(self.s.upper_bound, t):
+            return t
         else:
             return self.default(self.s)
 
@@ -562,6 +567,10 @@ def visit_tuple_type(self, t: TupleType) -> ProperType:
             assert isinstance(fallback, Instance)
             items = self.join_tuples(self.s, t)
             if items is not None:
+                if len(items) == 1 and isinstance(item := items[0], UnpackType):
+                    if isinstance(unpacked := get_proper_type(item.type), Instance):
+                        # Avoid double-wrapping tuple[*tuple[X, ...]]
+                        return unpacked
                 return TupleType(items, fallback)
             else:
                 # TODO: should this be a default fallback behaviour like for meet?

mypy/subtypes.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from collections.abc import Iterator
+from collections.abc import Iterable, Iterator
 from contextlib import contextmanager
 from typing import Any, Callable, Final, TypeVar, cast
 from typing_extensions import TypeAlias as _TypeAlias
@@ -414,6 +414,9 @@ def _is_subtype(self, left: Type, right: Type) -> bool:
             return is_proper_subtype(left, right, subtype_context=self.subtype_context)
         return is_subtype(left, right, subtype_context=self.subtype_context)
 
+    def _all_subtypes(self, lefts: Iterable[Type], rights: Iterable[Type]) -> bool:
+        return all(self._is_subtype(li, ri) for (li, ri) in zip(lefts, rights))
+
     # visit_x(left) means: is left (which is an instance of X) a subtype of right?
 
     def visit_unbound_type(self, left: UnboundType) -> bool:
@@ -856,11 +859,25 @@ def variadic_tuple_subtype(self, left: TupleType, right: TupleType) -> bool:
                 # There are some items on the left that will never have a matching length
                 # on the right.
                 return False
+            left_prefix = left_unpack_index
+            left_suffix = len(left.items) - left_prefix - 1
             left_unpack = left.items[left_unpack_index]
             assert isinstance(left_unpack, UnpackType)
             left_unpacked = get_proper_type(left_unpack.type)
             if not isinstance(left_unpacked, Instance):
-                # *Ts unpacks can't be split.
+                # *Ts unpack can't be split, except if it is all mapped to Anys or objects.
+                if self.is_top_type(right_item):
+                    right_prefix_types, middle, right_suffix_types = split_with_prefix_and_suffix(
+                        tuple(right.items), left_prefix, left_suffix
+                    )
+                    if not all(
+                        self.is_top_type(ri) or isinstance(ri, UnpackType) for ri in middle
+                    ):
+                        return False
+                    # Also check the tails match as well.
+                    return self._all_subtypes(
+                        left.items[:left_prefix], right_prefix_types
+                    ) and self._all_subtypes(left.items[-left_suffix:], right_suffix_types)
                 return False
             assert left_unpacked.type.fullname == "builtins.tuple"
             left_item = left_unpacked.args[0]
@@ -871,8 +888,6 @@ def variadic_tuple_subtype(self, left: TupleType, right: TupleType) -> bool:
             # and then check subtyping for all finite overlaps.
             if not self._is_subtype(left_item, right_item):
                 return False
-            left_prefix = left_unpack_index
-            left_suffix = len(left.items) - left_prefix - 1
             max_overlap = max(0, right_prefix - left_prefix, right_suffix - left_suffix)
             for overlap in range(max_overlap + 1):
                 repr_items = left.items[:left_prefix] + [left_item] * overlap
@@ -883,6 +898,11 @@ def variadic_tuple_subtype(self, left: TupleType, right: TupleType) -> bool:
                     return False
             return True
 
+    def is_top_type(self, typ: Type) -> bool:
+        if not self.proper_subtype and isinstance(get_proper_type(typ), AnyType):
+            return True
+        return is_named_instance(typ, "builtins.object")
+
     def visit_typeddict_type(self, left: TypedDictType) -> bool:
         right = self.right
         if isinstance(right, Instance):
@@ -1653,17 +1673,18 @@ def are_parameters_compatible(
         return True
     trivial_suffix = is_trivial_suffix(right) and not is_proper_subtype
 
+    trivial_vararg_suffix = False
     if (
-        right.arg_kinds == [ARG_STAR]
-        and isinstance(get_proper_type(right.arg_types[0]), AnyType)
+        right.arg_kinds[-1:] == [ARG_STAR]
+        and isinstance(get_proper_type(right.arg_types[-1]), AnyType)
         and not is_proper_subtype
+        and all(k.is_positional(star=True) for k in left.arg_kinds)
     ):
         # Similar to how (*Any, **Any) is considered a supertype of all callables, we consider
         # (*Any) a supertype of all callables with positional arguments. This is needed in
         # particular because we often refuse to try type inference if actual type is not
         # a subtype of erased template type.
-        if all(k.is_positional() for k in left.arg_kinds) and ignore_pos_arg_names:
-            return True
+        trivial_vararg_suffix = True
 
     # Match up corresponding arguments and check them for compatibility. In
     # every pair (argL, argR) of corresponding arguments from L and R, argL must
@@ -1697,7 +1718,11 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
             return not allow_partial_overlap and not trivial_suffix
         return not is_compat(right_arg.typ, left_arg.typ)
 
-    if _incompatible(left_star, right_star) or _incompatible(left_star2, right_star2):
+    if (
+        _incompatible(left_star, right_star)
+        and not trivial_vararg_suffix
+        or _incompatible(left_star2, right_star2)
+    ):
         return False
 
     # Phase 1b: Check non-star args: for every arg right can accept, left must
@@ -1727,8 +1752,8 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
     # Phase 1c: Check var args. Right has an infinite series of optional positional
     #           arguments. Get all further positional args of left, and make sure
     #           they're more general than the corresponding member in right.
-    # TODO: are we handling UnpackType correctly here?
-    if right_star is not None:
+    # TODO: handle suffix in UnpackType (i.e. *args: *Tuple[Ts, X, Y]).
+    if right_star is not None and not trivial_vararg_suffix:
         # Synthesize an anonymous formal argument for the right
         right_by_position = right.try_synthesizing_arg_from_vararg(None)
         assert right_by_position is not None

mypy/test/testtypes.py

@@ -1021,7 +1021,7 @@ def test_variadic_tuple_joins(self) -> None:
         self.assert_join(
             self.tuple(self.fx.a, self.fx.a),
             self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a]))),
-            self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a]))),
+            Instance(self.fx.std_tuplei, [self.fx.a]),
         )
         self.assert_join(
             self.tuple(self.fx.a, self.fx.a),
@@ -1049,12 +1049,12 @@ def test_variadic_tuple_joins(self) -> None:
             self.tuple(
                 self.fx.a, UnpackType(Instance(self.fx.std_tuplei, [self.fx.a])), self.fx.a
             ),
-            self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a]))),
+            Instance(self.fx.std_tuplei, [self.fx.a]),
         )
         self.assert_join(
             self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a]))),
             self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a]))),
-            self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a]))),
+            Instance(self.fx.std_tuplei, [self.fx.a]),
         )
         self.assert_join(
             self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a])), self.fx.a),
@@ -1584,11 +1584,12 @@ def make_call(*items: tuple[str, str | None]) -> CallExpr:
 class TestExpandTypeLimitGetProperType(TestCase):
     # WARNING: do not increase this number unless absolutely necessary,
     # and you understand what you are doing.
-    ALLOWED_GET_PROPER_TYPES = 9
+    ALLOWED_GET_PROPER_TYPES = 7
 
     @skipUnless(mypy.expandtype.__file__.endswith(".py"), "Skip for compiled mypy")
     def test_count_get_proper_type(self) -> None:
         with open(mypy.expandtype.__file__) as f:
             code = f.read()
-        get_proper_type_count = len(re.findall("get_proper_type", code))
+        get_proper_type_count = len(re.findall(r"get_proper_type\(", code))
+        get_proper_type_count -= len(re.findall(r"get_proper_type\(\)", code))
         assert get_proper_type_count == self.ALLOWED_GET_PROPER_TYPES

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

@@ -2309,18 +2309,21 @@ def higher_order(f: _CallableValue) -> None: ...
 def good1(*args: int) -> None: ...
 def good2(*args: str) -> int: ...
 
-def bad1(a: str, b: int, /) -> None: ...
-def bad2(c: bytes, *args: int) -> str: ...
-def bad3(*, d: str) -> int: ...
-def bad4(**kwargs: None) -> None: ...
+# These are special-cased for *args: Any (as opposite to *args: object)
+def ok1(a: str, b: int, /) -> None: ...
+def ok2(c: bytes, *args: int) -> str: ...
+
+def bad1(*, d: str) -> int: ...
+def bad2(**kwargs: None) -> None: ...
 
 higher_order(good1)
 higher_order(good2)
 
-higher_order(bad1)  # E: Argument 1 to "higher_order" has incompatible type "Callable[[str, int], None]"; expected "Callable[[VarArg(Any)], Any]"
-higher_order(bad2)  # E: Argument 1 to "higher_order" has incompatible type "Callable[[bytes, VarArg(int)], str]"; expected "Callable[[VarArg(Any)], Any]"
-higher_order(bad3)  # E: Argument 1 to "higher_order" has incompatible type "Callable[[NamedArg(str, 'd')], int]"; expected "Callable[[VarArg(Any)], Any]"
-higher_order(bad4)  # E: Argument 1 to "higher_order" has incompatible type "Callable[[KwArg(None)], None]"; expected "Callable[[VarArg(Any)], Any]"
+higher_order(ok1)
+higher_order(ok2)
+
+higher_order(bad1)  # E: Argument 1 to "higher_order" has incompatible type "Callable[[NamedArg(str, 'd')], int]"; expected "Callable[[VarArg(Any)], Any]"
+higher_order(bad2)  # E: Argument 1 to "higher_order" has incompatible type "Callable[[KwArg(None)], None]"; expected "Callable[[VarArg(Any)], Any]"
 [builtins fixtures/tuple.pyi]
 
 [case testAliasToCallableWithUnpack2]
@@ -2517,3 +2520,89 @@ x4: Foo[Unpack[tuple[str, ...]]]
 y4: Foo[Unpack[tuple[int, int]]]
 x4 is y4  # E: Non-overlapping identity check (left operand type: "Foo[Unpack[Tuple[str, ...]]]", right operand type: "Foo[int, int]")
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleErasureNormalized]
+from typing import TypeVarTuple, Unpack, Generic, Union
+from collections.abc import Callable
+
+Args = TypeVarTuple("Args")
+
+class Built(Generic[Unpack[Args]]):
+    pass
+
+def example(
+    fn: Union[Built[Unpack[Args]], Callable[[Unpack[Args]], None]]
+) -> Built[Unpack[Args]]: ...
+
+@example
+def command() -> None:
+    return
+reveal_type(command)  # N: Revealed type is "__main__.Built[()]"
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleSelfMappedPrefix]
+from typing import TypeVarTuple, Generic, Unpack
+
+Ts = TypeVarTuple("Ts")
+class Base(Generic[Unpack[Ts]]):
+    attr: tuple[Unpack[Ts]]
+
+    @property
+    def prop(self) -> tuple[Unpack[Ts]]:
+        return self.attr
+
+    def meth(self) -> tuple[Unpack[Ts]]:
+        return self.attr
+
+Ss = TypeVarTuple("Ss")
+class Derived(Base[str, Unpack[Ss]]):
+    def test(self) -> None:
+        reveal_type(self.attr)  # N: Revealed type is "Tuple[builtins.str, Unpack[Ss`1]]"
+        reveal_type(self.prop)  # N: Revealed type is "Tuple[builtins.str, Unpack[Ss`1]]"
+        reveal_type(self.meth())  # N: Revealed type is "Tuple[builtins.str, Unpack[Ss`1]]"
+[builtins fixtures/property.pyi]
+
+[case testTypeVarTupleProtocolPrefix]
+from typing import Protocol, Unpack, TypeVarTuple
+
+Ts = TypeVarTuple("Ts")
+class A(Protocol[Unpack[Ts]]):
+    def f(self, z: str, *args: Unpack[Ts]) -> None: ...
+
+class C:
+    def f(self, z: str, x: int) -> None: ...
+
+def f(x: A[Unpack[Ts]]) -> tuple[Unpack[Ts]]: ...
+
+reveal_type(f(C()))  # N: Revealed type is "Tuple[builtins.int]"
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleHomogeneousCallableNormalized]
+from typing import Generic, Unpack, TypeVarTuple
+
+Ts = TypeVarTuple("Ts")
+class C(Generic[Unpack[Ts]]):
+    def foo(self, *args: Unpack[Ts]) -> None: ...
+
+c: C[Unpack[tuple[int, ...]]]
+reveal_type(c.foo)  # N: Revealed type is "def (*args: builtins.int)"
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleJoinInstanceTypeVar]
+from typing import Any, Unpack, TypeVarTuple, TypeVar
+
+T = TypeVar("T")
+Ts = TypeVarTuple("Ts")
+
+def join(x: T, y: T) -> T: ...
+def test(xs: tuple[Unpack[Ts]], xsi: tuple[int, Unpack[Ts]]) -> None:
+    a: tuple[Any, ...]
+    reveal_type(join(xs, a))  # N: Revealed type is "builtins.tuple[Any, ...]"
+    reveal_type(join(a, xs))  # N: Revealed type is "builtins.tuple[Any, ...]"
+    aa: tuple[Unpack[tuple[Any, ...]]]
+    reveal_type(join(xs, aa))  # N: Revealed type is "builtins.tuple[Any, ...]"
+    reveal_type(join(aa, xs))  # N: Revealed type is "builtins.tuple[Any, ...]"
+    ai: tuple[int, Unpack[tuple[Any, ...]]]
+    reveal_type(join(xsi, ai))  # N: Revealed type is "Tuple[builtins.int, Unpack[builtins.tuple[Any, ...]]]"
+    reveal_type(join(ai, xsi))  # N: Revealed type is "Tuple[builtins.int, Unpack[builtins.tuple[Any, ...]]]"
+[builtins fixtures/tuple.pyi]