7060926: Attr.PostAttrAnalyzer misses a case

Reviewed-by: mcimadamore

langtools/src/share/classes/com/sun/tools/javac/comp/Attr.java

@@ -3387,6 +3387,13 @@ public class Attr extends JCTree.Visitor {
             super.visitNewClass(that);
         }
 
+        @Override
+        public void visitAssignop(JCAssignOp that) {
+            if (that.operator == null)
+                that.operator = new OperatorSymbol(names.empty, syms.unknownType, -1, syms.noSymbol);
+            super.visitAssignop(that);
+        }
+
         @Override
         public void visitBinary(JCBinary that) {
             if (that.operator == null)

langtools/test/tools/javac/failover/FailOver15.java

@@ -0,0 +1,19 @@
+/*
+ * @test /nodynamiccopyright/
+ * @bug 6970584 7060926
+ * @summary Attr.PostAttrAnalyzer misses a case
+ *
+ * @compile/fail/ref=FailOver15.out -XDrawDiagnostics -XDshouldStopPolicy=FLOW -XDdev FailOver15.java
+ */
+
+class Test {
+    void m() {
+        new UnknownClass<String, Void>() {
+            public String getString() {
+                String s = "";
+                s += "more";
+                return s;
+            }
+        }
+    }
+}

langtools/test/tools/javac/failover/FailOver15.out

@@ -0,0 +1,4 @@
+FailOver15.java:17:10: compiler.err.expected: ';'
+FailOver15.java:11:13: compiler.err.cant.resolve.location: kindname.class, UnknownClass, , , (compiler.misc.location: kindname.class, Test, null)
+2 errors
+