stubtest: flag redundant @disjoint_base decorators (#19715)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Author: JelleZijlstra

mypy/stubtest.py

@@ -506,28 +506,57 @@ def _is_disjoint_base(typ: type[object]) -> bool:
 def _verify_disjoint_base(
     stub: nodes.TypeInfo, runtime: type[object], object_path: list[str]
 ) -> Iterator[Error]:
-    # If it's final, doesn't matter whether it's a disjoint base or not
-    if stub.is_final:
-        return
     is_disjoint_runtime = _is_disjoint_base(runtime)
     # 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__"):
+    if (
+        is_disjoint_runtime
+        and not stub.is_disjoint_base
+        and not runtime.__dict__.get("__slots__")
+        and not stub.is_final
+        and not (stub.is_enum and stub.enum_members)
+    ):
         yield Error(
             object_path,
             "is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub",
             stub,
             runtime,
             stub_desc=repr(stub),
         )
-    elif not is_disjoint_runtime and stub.is_disjoint_base:
-        yield Error(
-            object_path,
-            "is marked with @disjoint_base in the stub, but isn't a disjoint base at runtime",
-            stub,
-            runtime,
-            stub_desc=repr(stub),
-        )
+    elif stub.is_disjoint_base:
+        if not is_disjoint_runtime:
+            yield Error(
+                object_path,
+                "is marked with @disjoint_base in the stub, but isn't a disjoint base at runtime",
+                stub,
+                runtime,
+                stub_desc=repr(stub),
+            )
+        if runtime.__dict__.get("__slots__"):
+            yield Error(
+                object_path,
+                "is marked as @disjoint_base, but also has slots; add __slots__ instead",
+                stub,
+                runtime,
+                stub_desc=repr(stub),
+            )
+        elif stub.is_final:
+            yield Error(
+                object_path,
+                "is marked as @disjoint_base, but also marked as @final; remove @disjoint_base",
+                stub,
+                runtime,
+                stub_desc=repr(stub),
+            )
+        elif stub.is_enum and stub.enum_members:
+            yield Error(
+                object_path,
+                "is marked as @disjoint_base, but is an enum with members, which is implicitly final; "
+                "remove @disjoint_base",
+                stub,
+                runtime,
+                stub_desc=repr(stub),
+            )
 
 
 def _verify_metaclass(

mypy/test/teststubtest.py

@@ -1407,14 +1407,9 @@ def spam(x=Flags4(0)): pass
             stub="""
             import sys
             from typing import Final, Literal
-            from typing_extensions import disjoint_base
-            if sys.version_info >= (3, 12):
-                class BytesEnum(bytes, enum.Enum):
-                    a = b'foo'
-            else:
-                @disjoint_base
-                class BytesEnum(bytes, enum.Enum):
-                    a = b'foo'
+            class BytesEnum(bytes, enum.Enum):
+                a = b'foo'
+
             FOO: Literal[BytesEnum.a]
             BAR: Final = BytesEnum.a
             BAZ: BytesEnum
@@ -1698,6 +1693,53 @@ def __next__(self) -> object: ...
             """,
             error=None,
         )
+        yield Case(
+            runtime="""
+            class IsDisjointBaseBecauseItHasSlots:
+                __slots__ = ("a",)
+                a: int
+            """,
+            stub="""
+            from typing_extensions import disjoint_base
+
+            @disjoint_base
+            class IsDisjointBaseBecauseItHasSlots:
+                a: int
+            """,
+            error="test_module.IsDisjointBaseBecauseItHasSlots",
+        )
+        yield Case(
+            runtime="""
+            class IsFinalSoDisjointBaseIsRedundant: ...
+            """,
+            stub="""
+            from typing_extensions import disjoint_base, final
+
+            @final
+            @disjoint_base
+            class IsFinalSoDisjointBaseIsRedundant: ...
+            """,
+            error="test_module.IsFinalSoDisjointBaseIsRedundant",
+        )
+        yield Case(
+            runtime="""
+            import enum
+
+            class IsEnumWithMembersSoDisjointBaseIsRedundant(enum.Enum):
+                A = 1
+                B = 2
+            """,
+            stub="""
+            from typing_extensions import disjoint_base
+            import enum
+
+            @disjoint_base
+            class IsEnumWithMembersSoDisjointBaseIsRedundant(enum.Enum):
+                A = 1
+                B = 2
+            """,
+            error="test_module.IsEnumWithMembersSoDisjointBaseIsRedundant",
+        )
 
     @collect_cases
     def test_has_runtime_final_decorator(self) -> Iterator[Case]: