Fix issue 17686: [REG2.075.0] Covariant return types doesn't work wit…

…h override in some cases

src/ddmd/mtype.d

@@ -775,7 +775,7 @@ extern (C++) abstract class Type : RootObject
 
                 // If t1n is forward referenced:
                 ClassDeclaration cd = (cast(TypeClass)t1n).sym;
-                if (cd.semanticRun < PASSsemanticdone)
+                if (cd.semanticRun < PASSsemanticdone && cd.baseok < BASEOKdone)
                     cd.semantic(null);
                 if (!cd.isBaseInfoComplete())
                 {

test/compilable/fix17686.d

@@ -0,0 +1,45 @@
+/* REQUIRED_ARGS:
+ * PERMUTE_ARGS:
+ */
+
+// https://issues.dlang.org/show_bug.cgi?id=17686
+
+interface INode
+{
+    @property INode parentNode();
+    @property IDocument ownerDocument();
+}
+interface IDocument: INode {}
+interface IEntityReference: INode {}
+
+class DOMImplementation(T)
+{
+    abstract class Node: INode
+    {
+        override
+        {
+            @property Node parentNode() { return null; }
+            @property Document ownerDocument() { return null; }
+        }
+
+        @property bool readonly() { return true; }
+    }
+    abstract class NodeWithChildren: Node {}
+
+    class Document: NodeWithChildren, IDocument {}
+
+    class EntityReference: NodeWithChildren, IEntityReference
+    {
+        override
+        {
+            @property bool readonly() { return true; }
+        }
+    }
+
+}
+
+void main()
+{
+	alias aaa = DOMImplementation!string;
+}
+