Throw errors in {{foo bar}}, foo not a function

Fix the “simpler helper” test and expand the cases where we detect that “foo” is missing or not a function (e.g. is a scalar property of the data context).

packages/blaze/lookup.js

@@ -52,14 +52,23 @@ Blaze.View.prototype.lookup = function (name, _options) {
     return wrapHelper(bindToCurrentDataIfIsFunction(UI._globalHelpers[name]));
   } else {
     return function () {
+      var isCalledAsFunction = (arguments.length > 0);
       var data = Blaze.getCurrentData();
-      if (lookupTemplate && ! (data && data[name]))
+      if (lookupTemplate && ! (data && data[name])) {
         throw new Error("No such template: " + name);
+      }
+      if (isCalledAsFunction && ! (data && data[name])) {
+        throw new Error("No such function: " + name);
+      }
       if (! data)
         return null;
       var x = data[name];
-      if (typeof x !== 'function')
+      if (typeof x !== 'function') {
+        if (isCalledAsFunction) {
+          throw new Error("Can't call non-function: " + x);
+        }
         return x;
+      }
       return x.apply(data, arguments);
     };
   }

packages/spacebars-tests/template_tests.js

@@ -39,17 +39,40 @@ Tinytest.add("spacebars-tests - template_tests - simple helper", function (test)
   tmpl.foo = 3;
   test.throws(function () {
     renderToDiv(tmpl);
-  });
+  }, /Can't call non-function/);
 
   delete tmpl.foo;
   test.throws(function () {
     renderToDiv(tmpl);
-  });
+  }, /No such function/);
 
   tmpl.foo = function () {};
   // doesn't throw
-  var div = renderToDiv(tmpl);
+  div = renderToDiv(tmpl);
   test.equal(canonicalizeHtml(div.innerHTML), '');
+
+  // now "foo" is a function in the data context
+  delete tmpl.foo;
+  R.set(1);
+  div = renderToDiv(tmpl, { foo: function (x) {
+    return x + R.get();
+  } });
+  test.equal(canonicalizeHtml(div.innerHTML), "124");
+  R.set(2);
+  Deps.flush();
+  test.equal(canonicalizeHtml(div.innerHTML), "125");
+
+  test.throws(function () {
+    renderToDiv(tmpl, {foo: 3});
+  }, /Can't call non-function/);
+
+  test.throws(function () {
+    renderToDiv(tmpl, {foo: null});
+  }, /No such function/);
+
+  test.throws(function () {
+    renderToDiv(tmpl, {});
+  }, /No such function/);
 });
 
 Tinytest.add("spacebars-tests - template_tests - dynamic template", function (test) {