correct semantics of __slots__ with inheritance (PY-5939)

python/src/com/jetbrains/python/inspections/PyUnresolvedReferencesInspection.java

@@ -154,10 +154,24 @@ public class PyUnresolvedReferencesInspection extends PyInspection {
           final PyClass pyClass = ((PyClassType)type).getPyClass();
           if (pyClass != null && pyClass.isNewStyleClass()) {
             final List<String> slots = pyClass.getSlots();
-            if (slots != null && !slots.contains(node.getReferencedName()) && !slots.contains("__dict__")) {
+            final String attrName = node.getReferencedName();
+            if (slots != null && !slots.contains(attrName) && !slots.contains(PyNames.DICT)) {
+              for (PyClassRef ref : pyClass.iterateAncestors()) {
+                final PyClass ancestor = ref.getPyClass();
+                if (ancestor == null) {
+                  return;
+                }
+                if (PyNames.OBJECT.equals(ancestor.getName())) {
+                  break;
+                }
+                final List<String> ancestorSlots = ancestor.getSlots();
+                if (ancestorSlots == null || ancestorSlots.contains(attrName) || ancestorSlots.contains(PyNames.DICT)) {
+                  return;
+                }
+              }
               final ASTNode nameNode = node.getNameElement();
               final PsiElement e = nameNode != null ? nameNode.getPsi() : node;
-              registerProblem(e, "'" + pyClass.getName() + "' object has no attribute '" + node.getReferencedName() + "'");
+              registerProblem(e, "'" + pyClass.getName() + "' object has no attribute '" + attrName + "'");
             }
           }
         }

python/testData/inspections/PyUnresolvedReferencesInspection/slotsSubclass.py

@@ -0,0 +1,15 @@
+class A(object):
+    __slots__ = ['a', 'b']
+    def __init__(self):
+        self.a = None # <- all ok here
+        self.b = None # <- all ok here
+
+class C(A):
+    __slots__ = ['c', 'd']
+
+    def __init__(self, c):
+        super(C, self).__init__()
+        self.c = c
+        self.d = self.b
+        if self.c:
+            self.a = 10

python/testSrc/com/jetbrains/python/inspections/PyUnresolvedReferencesInspectionTest.java

@@ -32,6 +32,10 @@ public class PyUnresolvedReferencesInspectionTest extends PyTestCase {
     doTest();
   }
 
+  public void testSlotsSubclass() {  // PY-5939
+    doTest();
+  }
+
   public void testImportExceptImportError() {
     doTest();
   }