stubtest: do not require @disjoint_base if there are __slots__ (python#19701)

Author: JelleZijlstra

mypy/stubtest.py

@@ -510,7 +510,9 @@ def _verify_disjoint_base(
     if stub.is_final:
         return
     is_disjoint_runtime = _is_disjoint_base(runtime)
-    if is_disjoint_runtime and not stub.is_disjoint_base:
+    # Don't complain about missing @disjoint_base if there are __slots__, because
+    # in that case we can infer that it's a disjoint base.
+    if is_disjoint_runtime and not stub.is_disjoint_base and not runtime.__dict__.get("__slots__"):
         yield Error(
             object_path,
             "is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub",

mypy/test/teststubtest.py

@@ -1430,6 +1430,31 @@ class BytesEnum(bytes, enum.Enum):
             """,
             error=None,
         )
+        yield Case(
+            stub="""
+            class HasSlotsAndNothingElse:
+                __slots__ = ("x",)
+                x: int
+
+            class HasInheritedSlots(HasSlotsAndNothingElse):
+                pass
+
+            class HasEmptySlots:
+                __slots__ = ()
+            """,
+            runtime="""
+            class HasSlotsAndNothingElse:
+                __slots__ = ("x",)
+                x: int
+
+            class HasInheritedSlots(HasSlotsAndNothingElse):
+                pass
+
+            class HasEmptySlots:
+                __slots__ = ()
+            """,
+            error=None,
+        )
 
     @collect_cases
     def test_decorator(self) -> Iterator[Case]: