Prioritize .then on thenable functios (#735)

lib/dust.js

@@ -248,16 +248,25 @@
   };
 
   /**
-   * Decide somewhat-naively if something is a Thenable.
+   * Decide somewhat-naively if something is a Thenable.  Matches Promises A+ Spec, section 1.2 “thenable” is an object or function that defines a then method."
    * @param elem {*} object to inspect
    * @return {Boolean} is `elem` a Thenable?
    */
   dust.isThenable = function(elem) {
-    return elem &&
-           typeof elem === 'object' &&
+    return elem &&  /* Beware: `typeof null` is `object` */
+           (typeof elem === 'object' || typeof elem === 'function') &&
            typeof elem.then === 'function';
   };
 
+  /**
+   * Decide if an element is a function but not Thenable; it is prefereable to resolve a thenable function by its `.then` method.
+   * @param elem {*} target of inspection
+   * @return {Boolean} is `elem` a function without a `.then` property?
+   */
+  dust.isNonThenableFunction = function(elem) {
+    return typeof elem === 'function' && !dust.isThenable(elem);
+  };
+
   /**
    * Decide very naively if something is a Stream.
    * @param elem {*} object to inspect
@@ -430,7 +439,7 @@
       }
     }
 
-    if (typeof ctx === 'function') {
+    if (dust.isNonThenableFunction(ctx)) {
       fn = function() {
         try {
           return ctx.apply(ctxThis, arguments);
@@ -747,7 +756,7 @@
   };
 
   Chunk.prototype.reference = function(elem, context, auto, filters) {
-    if (typeof elem === 'function') {
+    if (dust.isNonThenableFunction(elem)) {
       elem = elem.apply(context.current(), [this, context, null, {auto: auto, filters: filters}]);
       if (elem instanceof Chunk) {
         return elem;
@@ -772,7 +781,7 @@
         chunk = this,
         i, len, head;
 
-    if (typeof elem === 'function' && !dust.isTemplateFn(elem)) {
+    if (dust.isNonThenableFunction(elem) && !dust.isTemplateFn(elem)) {
       try {
         elem = elem.apply(context.current(), [this, context, bodies, params]);
       } catch(err) {

test/templates/all.js

@@ -24,6 +24,16 @@ function FalsePromise(err, data) {
   return defer.promise;
 }
 
+/**
+  * A naive function factory that adds a thenable that resolves to the given value.
+  * @param resolvesTo {*} The value passed to the `resolve` then argument
+*/
+function createThenableFunction(resolvesTo) {
+  var fn = function () {};
+  fn.then = function (res) { res(resolvesTo) };
+  return fn;
+}
+
 /**
  * A naive Stream constructor that streams the provided array asynchronously
  * @param arr {Array<Object|Error>|String} items to be streamed
@@ -914,6 +924,20 @@ return [
 		expected: "Eventually magic!",
 		message: "should reserve an async section for a thenable returned from a function"
 	  },
+    {
+      name:     "thenable section from thenable function",
+  		source:   "{#thenable}Eventually poof!{/thenable}",
+  		context: { "thenable": createThenableFunction("poof!") },
+  		expected: "Eventually poof!",
+  		message: "should reserve an async section for a thenable function"
+    },
+    {
+      name:     "thenable reference from thenable function",
+  		source:   "A {thenable} thing",
+  		context: { "thenable": createThenableFunction("real") },
+  		expected: "A real thing",
+  		message: "should reserve an async reference for a thenable function"
+    },
 	  {
 		name:     "thenable deep section",
 		source:   "Eventually my {#magic.ally}{delicious}{/magic.ally} will come",