deal with UnhandledPromiseRejectionWarning

promise now returns the promise, if it is one

must.js

@@ -1184,7 +1184,7 @@ defineGetter(Must.prototype, "reject", function() {
 })
 
 /**
- * Assert that an object is a promise.
+ * Assert that an object is a promise, and returns it.
  *
  * The determination uses duck typing, i.e., it checks whether the object has
  * a `then` and a `catch` method.
@@ -1213,6 +1213,7 @@ defineGetter(Must.prototype, "reject", function() {
  */
 Must.prototype.promise = function() {
   this.assert(isPromise(this.actual), isPromiseMsg, { actual: this.actual })
+  return this.actual
 }
 
 /**

test/must/promise_test.js

@@ -14,7 +14,25 @@ describe("Must.prototype.promise", function() {
     assert.pass(function () { Must(Promise.resolve(42)).be.promise() })
   })
 
-  it("must pass given a Promise implementation, with a rejected promise", function () {
-    assert.pass(function () { Must(Promise.reject(new Error())).be.promise() })
+  it("must pass given a Promise implementation, with a rejected promise (and passes it through)", function(done) {
+    var p = Promise.reject(new Error())
+    assert.pass(function() { Must(p).be.promise() })
+    p.catch(function() { done() }) // deal with UnhandledPromiseRejectionWarning
+  })
+
+  it("passes through a resolved promise", function() {
+    var p = Promise.resolve(42)
+    assert(Must(p).be.promise() === p)
+  })
+
+  it("passes through a rejected promise", function(done) {
+    var rejection = new Error()
+    var p = Promise.reject(rejection)
+    var outcome = Must(p).be.promise()
+    assert(outcome === p)
+    p.catch(function(err) { // deal with UnhandledPromiseRejectionWarning
+      assert(err === rejection)
+      done()
+    })
   })
 })