Ensure subtyping works with erased typevartuples in callables

mypy/subtypes.py

@@ -1620,6 +1620,18 @@ def are_parameters_compatible(
         return True
     trivial_suffix = is_trivial_suffix(right) and not is_proper_subtype
 
+    # def _(*a: Unpack[tuple[Any, ...]]) allows any number of arguments, not just infinite.
+    if right_star and isinstance(right_star.typ, UnpackType):
+        right_star_inner_type = get_proper_type(right_star.typ.type)
+        trivial_varargs = (
+            isinstance(right_star_inner_type, Instance)
+            and right_star_inner_type.type.fullname == "builtins.tuple"
+            and len(right_star_inner_type.args) == 1
+            and isinstance(get_proper_type(right_star_inner_type.args[0]), AnyType)
+        )
+    else:
+        trivial_varargs = False
+
     if (
         right.arg_kinds == [ARG_STAR]
         and isinstance(get_proper_type(right.arg_types[0]), AnyType)
@@ -1657,14 +1669,16 @@ def are_parameters_compatible(
     #           Furthermore, if we're checking for compatibility in all cases,
     #           we confirm that if R accepts an infinite number of arguments,
     #           L must accept the same.
-    def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | None) -> bool:
+    def _incompatible(
+        left_arg: FormalArgument | None, right_arg: FormalArgument | None, varargs: bool
+    ) -> bool:
         if right_arg is None:
             return False
         if left_arg is None:
-            return not allow_partial_overlap and not trivial_suffix
+            return not (allow_partial_overlap or trivial_suffix or (varargs and trivial_varargs))
         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, True) or _incompatible(left_star2, right_star2, False):
         return False
 
     # Phase 1b: Check non-star args: for every arg right can accept, left must
@@ -1690,7 +1704,7 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
     #           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 and not trivial_suffix:
+    if right_star is not None and not trivial_suffix and not trivial_varargs:
         # 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

test-data/unit/check-generics.test

@@ -3548,3 +3548,29 @@ def foo(x: T):
     reveal_type(C)  # N: Revealed type is "Overload(def [T, S] (x: builtins.int, y: S`-1) -> __main__.C[__main__.Int[S`-1]], def [T, S] (x: builtins.str, y: S`-1) -> __main__.C[__main__.Str[S`-1]])"
     reveal_type(C(0, x))  # N: Revealed type is "__main__.C[__main__.Int[T`-1]]"
     reveal_type(C("yes", x))  # N: Revealed type is "__main__.C[__main__.Str[T`-1]]"
+
+[case testTypeVarTupleInCallableInUnion]
+from typing import TypeVarTuple, Unpack, Generic, Union
+from collections.abc import Callable
+
+Args = TypeVarTuple("Args")
+
+class Built(Generic[Unpack[Args]]):
+    pass
+
+def example(n: Union[Built[Unpack[Args]], Callable[[Unpack[Args]], None]]) -> Built[Unpack[Args]]: ...
+
+reveal_type(example)  # N: Revealed type is "def [Args] (n: Union[__main__.Built[Unpack[Args`-1]], def (*Unpack[Args`-1])]) -> __main__.Built[Unpack[Args`-1]]"
+
+@example
+def command1() -> None:
+    return
+
+reveal_type(command1)  # N: Revealed type is "__main__.Built[()]"
+
+@example
+def command2(a: int) -> None:
+    return
+
+reveal_type(command2)  # N: Revealed type is "__main__.Built[builtins.int]"
+[builtins fixtures/tuple.pyi]