Normalize coroutine() return to Promise type

lib/functions.php

@@ -734,7 +734,13 @@ function wait(Promise $promise) {
 function coroutine(callable $func) {
     return function () use ($func) {
         $out = \call_user_func_array($func, \func_get_args());
-        return ($out instanceof \Generator) ? resolve($out) : $out;
+        if ($out instanceof \Generator) {
+            return resolve($out);
+        } elseif ($out instanceof Promise) {
+            return $out;
+        } else {
+            return new Success($out);
+        }
     };
 }
 

test/FunctionsTest.php

@@ -671,6 +671,30 @@ class FunctionsTest extends \PHPUnit_Framework_TestCase {
         $this->assertSame(1, $invoked);
     }
 
+    public function testCoroutineWrapsNonGeneratorReturnInPromise() {
+        $co = \Amp\coroutine(function () {
+            return 42;
+        });
+        $out = $co();
+        $this->assertInstanceOf("\Amp\Success", $out);
+        $invoked = false;
+        $out->when(function ($error, $result) use (&$invoked) {
+            $this->assertNull($error);
+            $this->assertSame(42, $result);
+            $invoked = true;
+        });
+        $this->assertTrue($invoked);
+    }
+
+    public function testCoroutineReturnsPromiseResultUnmodified() {
+        $success = new \Amp\Success;
+        $co = \Amp\coroutine(function () use ($success) {
+            return $success;
+        });
+        $out = $co();
+        $this->assertSame($success, $out);
+    }
+
     public function testNestedCoroutineResolutionContinuation() {
         $invoked = 0;
         \Amp\run(function () use (&$invoked) {