raise type error if not possible to get a consistent method resolution order

Author: tomasstupka

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/type/TypeNodes.java

@@ -52,6 +52,7 @@
 
 import com.oracle.graal.python.PythonLanguage;
 import com.oracle.graal.python.builtins.PythonBuiltinClassType;
+import static com.oracle.graal.python.builtins.PythonBuiltinClassType.TypeError;
 import com.oracle.graal.python.builtins.objects.PNone;
 import com.oracle.graal.python.builtins.objects.cext.CExtNodes;
 import com.oracle.graal.python.builtins.objects.cext.CExtNodes.GetTypeMemberNode;
@@ -773,10 +774,22 @@ private static PythonAbstractClass[] mergeMROs(MROMergeState[] toMerge, List<Pyt
                 idx = -1;
             }
 
+            List<PythonAbstractClass> notMerged = new ArrayList<>();
             for (MROMergeState mergee : toMerge) {
                 if (!mergee.isMerged()) {
-                    throw new IllegalStateException();
+                    PythonAbstractClass candidate = mergee.getCandidate();
+                    if (!notMerged.contains(candidate)) {
+                        notMerged.add(candidate);
+                    }
+                }
+            }
+            if (!notMerged.isEmpty()) {
+                Iterator<PythonAbstractClass> it = notMerged.iterator();
+                StringBuilder bases = new StringBuilder(GetNameNode.doSlowPath(it.next()));
+                while (it.hasNext()) {
+                    bases.append(", ").append(GetNameNode.doSlowPath(it.next()));
                 }
+                throw PRaiseNode.getUncached().raise(TypeError, ErrorMessages.CANNOT_GET_CONSISTEMT_METHOD_RESOLUTION, bases);
             }
 
             return mro.toArray(new PythonAbstractClass[mro.size()]);

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/ErrorMessages.java

@@ -117,6 +117,7 @@ public abstract class ErrorMessages {
     public static final String CANNOT_CREATE_WEAK_REFERENCE_TO = "cannot create weak reference to '%p' object";
     public static final String CANNOT_DELETE_ATTRIBUTE = "can't delete %s.%s";
     public static final String CANNOT_GET_SHAPE_OF_NATIVE_CLS = "cannot get shape of native class";
+    public static final String CANNOT_GET_CONSISTEMT_METHOD_RESOLUTION = "Cannot create a consistent method resolution order (MRO) for bases %p, %p";
     public static final String CANNOT_HANDLE_ZIP_FILE = "cannot handle Zip file: '%s'";
     public static final String CANNOT_IMPORT_NAME = "cannot import name '%s'";
     public static final String CANNOT_INITIALIZE_WITH = "cannot initialize %s with %s%s";