detect slots layout conflict in multiple inheritance

pyrefly/lib/alt/class/class_metadata.rs

@@ -402,6 +402,35 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         }
         let extends_abc = self.extends_abc(&bases_with_metadata, metaclass);
 
+        // Check for __slots__ layout conflict: if two or more base classes
+        // define non-empty __slots__, CPython raises TypeError at runtime
+        // ("multiple bases have instance lay-out conflict").
+        {
+            let bases_with_nonempty_slots: Vec<&Class> = bases_with_metadata
+                .iter()
+                .filter(|(_, metadata)| {
+                    metadata.slots_info().is_some_and(|si| !si.names.is_empty())
+                })
+                .map(|(base, _)| base)
+                .collect();
+            if bases_with_nonempty_slots.len() > 1 {
+                let names: Vec<String> = bases_with_nonempty_slots
+                    .iter()
+                    .map(|b| format!("`{}`", b.name()))
+                    .collect();
+                self.error(
+                    errors,
+                    cls.range(),
+                    ErrorInfo::Kind(ErrorKind::InvalidInheritance),
+                    format!(
+                        "Class `{}` has multiple base classes with non-empty `__slots__` ({}), which causes a TypeError at runtime",
+                        cls.name(),
+                        names.join(", "),
+                    ),
+                );
+            }
+        }
+
         // Compute final base class list.
         let bases = if is_typed_dict && bases_with_metadata.is_empty() {
             // This is a "fallback" class that contains attributes that are available on all TypedDict subclasses.

pyrefly/lib/test/slots.rs

@@ -281,7 +281,6 @@ c.x = 2  # OK: descriptor __set__ handles this, not instance storage
 
 // https://github.com/facebook/pyrefly/issues/2917
 testcase!(
-    bug = "Should detect instance layout conflict when multiple bases have __slots__",
     test_slots_multiple_inheritance_layout_conflict,
     r#"
 class Left:
@@ -292,13 +291,12 @@ class Right:
 
 # Inheriting from two classes that both define non-empty __slots__
 # causes a TypeError at runtime.
-class Combined(Left, Right): ...
+class Combined(Left, Right): ...  # E: multiple base classes with non-empty `__slots__`
 "#,
 );
 
 // https://github.com/facebook/pyrefly/issues/2916
 testcase!(
-    bug = "Should detect instance layout conflict even with identical slot names",
     test_slots_layout_conflict_same_names,
     r#"
 class First:
@@ -308,7 +306,51 @@ class Second:
     __slots__ = ("x",)
 
 # Even though the slot names match, these are different C-level layouts.
-class Both(First, Second): ...
+class Both(First, Second): ...  # E: multiple base classes with non-empty `__slots__`
+"#,
+);
+
+testcase!(
+    test_slots_layout_conflict_empty_slots_ok,
+    r#"
+class A:
+    __slots__ = ("x",)
+
+class B:
+    __slots__ = ()
+
+# One base has non-empty slots, the other has empty slots - this is fine.
+class C(A, B): ...
+"#,
+);
+
+testcase!(
+    test_slots_layout_conflict_both_empty_ok,
+    r#"
+class A:
+    __slots__ = ()
+
+class B:
+    __slots__ = ()
+
+# Both bases have empty slots - no conflict.
+class C(A, B): ...
+"#,
+);
+
+testcase!(
+    test_slots_layout_conflict_three_bases,
+    r#"
+class A:
+    __slots__ = ("x",)
+
+class B:
+    __slots__ = ("y",)
+
+class C:
+    __slots__ = ("z",)
+
+class D(A, B, C): ...  # E: multiple base classes with non-empty `__slots__`
 "#,
 );