Properly handle unpacks in overlap checks (#17356)

Fixes #17319

This is still not 100% robust, but at least it should not crash, and
should cover correctly vast majority of cases.

mypy/meet.py

@@ -611,6 +611,19 @@ def are_tuples_overlapping(
     right = adjust_tuple(right, left) or right
     assert isinstance(left, TupleType), f"Type {left} is not a tuple"
     assert isinstance(right, TupleType), f"Type {right} is not a tuple"
+
+    # This algorithm works well if only one tuple is variadic, if both are
+    # variadic we may get rare false negatives for overlapping prefix/suffix.
+    # Also, this ignores empty unpack case, but it is probably consistent with
+    # how we handle e.g. empty lists in overload overlaps.
+    # TODO: write a more robust algorithm for cases where both types are variadic.
+    left_unpack = find_unpack_in_list(left.items)
+    right_unpack = find_unpack_in_list(right.items)
+    if left_unpack is not None:
+        left = expand_tuple_if_possible(left, len(right.items))
+    if right_unpack is not None:
+        right = expand_tuple_if_possible(right, len(left.items))
+
     if len(left.items) != len(right.items):
         return False
     return all(
@@ -624,6 +637,27 @@ def are_tuples_overlapping(
     )
 
 
+def expand_tuple_if_possible(tup: TupleType, target: int) -> TupleType:
+    if len(tup.items) > target + 1:
+        return tup
+    extra = target + 1 - len(tup.items)
+    new_items = []
+    for it in tup.items:
+        if not isinstance(it, UnpackType):
+            new_items.append(it)
+            continue
+        unpacked = get_proper_type(it.type)
+        if isinstance(unpacked, TypeVarTupleType):
+            instance = unpacked.tuple_fallback
+        else:
+            # Nested non-variadic tuples should be normalized at this point.
+            assert isinstance(unpacked, Instance)
+            instance = unpacked
+        assert instance.type.fullname == "builtins.tuple"
+        new_items.extend([instance.args[0]] * extra)
+    return tup.copy_modified(items=new_items)
+
+
 def adjust_tuple(left: ProperType, r: ProperType) -> TupleType | None:
     """Find out if `left` is a Tuple[A, ...], and adjust its length to `right`"""
     if isinstance(left, Instance) and left.type.fullname == "builtins.tuple":

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

@@ -1808,6 +1808,63 @@ def test(a: Tuple[int, str], b: Tuple[bool], c: Tuple[bool, ...]):
     reveal_type(add(b, c))  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
 [builtins fixtures/tuple.pyi]
 
+[case testTypeVarTupleOverloadOverlap]
+from typing import Union, overload, Tuple
+from typing_extensions import Unpack
+
+class Int(int): ...
+
+A = Tuple[int, Unpack[Tuple[int, ...]]]
+B = Tuple[int, Unpack[Tuple[str, ...]]]
+
+@overload
+def f(arg: A) -> int: ...
+@overload
+def f(arg: B) -> str: ...
+def f(arg: Union[A, B]) -> Union[int, str]:
+    ...
+
+A1 = Tuple[int, Unpack[Tuple[Int, ...]]]
+B1 = Tuple[Unpack[Tuple[Int, ...]], int]
+
+@overload
+def f1(arg: A1) -> int: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+@overload
+def f1(arg: B1) -> str: ...
+def f1(arg: Union[A1, B1]) -> Union[int, str]:
+    ...
+
+A2 = Tuple[int, int, int]
+B2 = Tuple[int, Unpack[Tuple[int, ...]]]
+
+@overload
+def f2(arg: A2) -> int: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+@overload
+def f2(arg: B2) -> str: ...
+def f2(arg: Union[A2, B2]) -> Union[int, str]:
+    ...
+
+A3 = Tuple[int, int, int]
+B3 = Tuple[int, Unpack[Tuple[str, ...]]]
+
+@overload
+def f3(arg: A3) -> int: ...
+@overload
+def f3(arg: B3) -> str: ...
+def f3(arg: Union[A3, B3]) -> Union[int, str]:
+    ...
+
+A4 = Tuple[int, int, Unpack[Tuple[int, ...]]]
+B4 = Tuple[int]
+
+@overload
+def f4(arg: A4) -> int: ...
+@overload
+def f4(arg: B4) -> str: ...
+def f4(arg: Union[A4, B4]) -> Union[int, str]:
+    ...
+[builtins fixtures/tuple.pyi]
+
 [case testTypeVarTupleIndexOldStyleNonNormalizedAndNonLiteral]
 from typing import Any, Tuple
 from typing_extensions import Unpack