Return null for the most derived class type if there is no total order among classes (PY-17841)

python/src/com/jetbrains/python/psi/types/PyClassTypeImpl.java

@@ -301,23 +301,31 @@ public class PyClassTypeImpl extends UserDataHolderBase implements PyClassType {
     if (classTypes.isEmpty()) {
       return null;
     }
-    return Collections.max(classTypes, new Comparator<PyClassLikeType>() {
-      @Override
-      public int compare(@Nullable PyClassLikeType t1, @Nullable PyClassLikeType t2) {
-        if (t1 == t2 || t1 != null && t1.equals(t2)) {
-          return 0;
+    try {
+      return Collections.max(classTypes, new Comparator<PyClassLikeType>() {
+        @Override
+        public int compare(@Nullable PyClassLikeType t1, @Nullable PyClassLikeType t2) {
+          if (t1 == t2 || t1 != null && t1.equals(t2)) {
+            return 0;
+          }
+          else if (t2 == null || t1 != null && Sets.newHashSet(t1.getAncestorTypes(context)).contains(t2)) {
+            return 1;
+          }
+          else if (t1 == null || Sets.newHashSet(t2.getAncestorTypes(context)).contains(t1)) {
+            return -1;
+          }
+          else {
+            throw new NotDerivedClassTypeException();
+          }
         }
-        if (t1 != null && Sets.newHashSet(t1.getAncestorTypes(context)).contains(t2)) {
-          return 1;
-        }
-        else if (t2 != null && Sets.newHashSet(t2.getAncestorTypes(context)).contains(t1)) {
-          return -1;
-        }
-        else {
-          return 0;
-        }
-      }
-    });
+      });
+    }
+    catch (NotDerivedClassTypeException ignored) {
+      return null;
+    }
+  }
+
+  private static final class NotDerivedClassTypeException extends RuntimeException {
   }
 
   private List<PyClassLikeType> getAllExplicitMetaClassTypes(@NotNull TypeEvalContext context) {

python/testData/inspections/PyUnresolvedReferencesInspection3K/noMostDerivedMetaClass.py

@@ -0,0 +1,24 @@
+class M1(type):
+    def foo(cls):
+        pass
+
+
+class M2(type):
+    def bar(cls):
+        pass
+
+
+class C1(metaclass=M1):
+    pass
+
+
+class C2(metaclass=M2):
+    pass
+
+
+class D(C1, C2):
+    pass
+
+
+print(D.<warning descr="Unresolved attribute reference 'foo' for class 'D'">foo</warning>()())
+print(D.<warning descr="Unresolved attribute reference 'bar' for class 'D'">bar</warning>())

python/testSrc/com/jetbrains/python/inspections/Py3UnresolvedReferencesInspectionTest.java

@@ -132,4 +132,9 @@ public class Py3UnresolvedReferencesInspectionTest extends PyTestCase {
   public void testMostDerivedMetaClass() {
     doTest();
   }
+
+  // PY-17841
+  public void testNoMostDerivedMetaClass() {
+    doTest();
+  }
 }