Spacebars: Disallow non-initial this in paths

Apparently `../this` used to work as a synonym for `..` in Handlebars, but in Spacebars it actually would look for a property named `this`, which caused confusing breaking changes in apps.

packages/spacebars-compiler/spacebars_tests.js

@@ -121,6 +121,12 @@ Tinytest.add("spacebars - stache tags", function (test) {
               ['NUMBER', 2, 'y']]});
   run('{{> foo x=1 y=2 z}}',
       "Can't have a non-keyword argument");
+
+  run('{{true.foo}}', "Can't use");
+  run('{{foo.this}}', "Can only use");
+  run('{{./this}}', "Can only use");
+  run('{{../this}}', "Can only use");
+
 });
 
 

packages/spacebars-compiler/templatetag.js

@@ -141,9 +141,13 @@ TemplateTag.parse = function (scannerOrString) {
         segments.push(seg);
       } else {
         var id = scanIdentifier(! segments.length);
-        if (id === 'this' && ! segments.length) {
-          // initial `this`
-          segments.push('.');
+        if (id === 'this') {
+          if (! segments.length) {
+            // initial `this`
+            segments.push('.');
+          } else {
+            error("Can only use `this` at the beginning of a path.\nInstead of `foo.this` or `../this`, just write `foo` or `..`.");
+          }
         } else {
           segments.push(id);
         }