Issue #2089 - Report unhandled rejection for optimized away promise

Based-on: m-c 1498775

js/src/builtin/Promise.cpp

@@ -2299,8 +2299,25 @@ RunResolutionFunction(JSContext *cx, HandleObject resolutionFun, HandleValue res
         return Call(cx, calleeOrRval, UndefinedHandleValue, result, &calleeOrRval);
     }
 
-    if (!promiseObj)
+    if (!promiseObj) {
+        if (mode == RejectMode) {
+            // The rejection will never be handled, given the returned promise
+            // is known to be unused, and already optimized away.
+            //
+            // Create temporary Promise object and reject it, in order to
+            // report the unhandled rejection.
+            //
+            // Allocation time points wrong time, but won't matter much.
+            Rooted<PromiseObject*> temporaryPromise(cx);
+            temporaryPromise = CreatePromiseObjectWithoutResolutionFunctions(cx);
+            if (!temporaryPromise) {
+                cx->clearPendingException();
+                return true;
+            }
+            return RejectPromiseInternal(cx, temporaryPromise, result);
+        }
         return true;
+    }
 
     Handle<PromiseObject*> promise = promiseObj.as<PromiseObject>();
     if (promise->state() != JS::PromiseState::Pending)

js/src/tests/non262/Promise/promise-rejection-tracking-optimized.js

@@ -0,0 +1,34 @@
+// |reftest| skip-if(!xulRuntime.shell) -- needs setPromiseRejectionTrackerCallback
+
+const UNHANDLED = 0;
+const HANDLED   = 1;
+
+let rejections = new Map();
+function rejectionTracker(promise, state) {
+  rejections.set(promise, state);
+}
+setPromiseRejectionTrackerCallback(rejectionTracker);
+
+// If the return value of then is not used, the promise object is optimized
+// away, but if a rejection happens, the rejection should be notified.
+Promise.resolve().then(() => { throw 1; });
+drainJobQueue();
+
+assertEq(rejections.size, 1);
+
+let [[promise, state]] = rejections;
+assertEq(state, UNHANDLED);
+
+let exc;
+promise.catch(x => { exc = x; });
+drainJobQueue();
+
+// we handled it after all
+assertEq(rejections.get(promise), HANDLED);
+
+// the right exception was reported
+assertEq(exc, 1);
+
+if (this.reportCompare) {
+  reportCompare(true,true);
+}