Added annotation to suppress errors like * claimed to be synchronous, but has invocations of synchronous methods. This is usesful in cases where the developer knows that the APIs are synchronous but might be mistakenly marked as async for some reason. This is a possible workaround for issues like https://github.com/konsoletyper/teavm/issues/248

core/src/main/java/org/teavm/model/util/AsyncMethodFinder.java

@@ -28,6 +28,7 @@ import org.teavm.callgraph.CallGraphNode;
 import org.teavm.callgraph.CallSite;
 import org.teavm.diagnostics.Diagnostics;
 import org.teavm.interop.Async;
+import org.teavm.interop.SuppressSyncErrors;
 import org.teavm.interop.Sync;
 import org.teavm.model.CallLocation;
 import org.teavm.model.ClassReader;
@@ -156,9 +157,17 @@ public class AsyncMethodFinder {
         }
         if (method.getAnnotations().get(Sync.class.getName()) != null
                 || method.getAnnotations().get(InjectedBy.class.getName()) != null) {
-            diagnostics.error(new CallLocation(methodRef), "Method {{m0}} is claimed to be synchronous, "
+            if (method.getAnnotations().get(SuppressSyncErrors.class.getName()) == null) {
-                    + "but it is has invocations of asynchronous methods:" + stack.toString(), methodRef);
+                diagnostics.error(new CallLocation(methodRef), "Method {{m0}} is claimed to be "
-            return;
+                        + "synchronous, but it is has invocations of asynchronous methods:" 
+                        + stack.toString(), methodRef);
+                return;
+            } else {
+                diagnostics.warning(new CallLocation(methodRef), "Error as Warning because "
+                        + " Method {{m0}} has @SuppressSyncErrors annoation. Method {{m0}} "
+                        + "is claimed to be synchronous, but it is has invocations of "
+                        + "asynchronous methods:" + stack.toString(), methodRef);
+            }
         }
         for (CallSite callSite : node.getCallerCallSites()) {
             MethodReference nextMethod = callSite.getCaller().getMethod();

interop/core/src/main/java/org/teavm/interop/SuppressSyncErrors.java

@@ -0,0 +1,28 @@
+/*
+ *  Copyright 2016 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.interop;
+
+import java.lang.annotation.*;
+
+/**
+ * 
+ * @author Steve Hannah
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.METHOD)
+public @interface SuppressSyncErrors {
+}
+