Differentiate between lambdas in the context (which should have mustache semantics) and helpers (which have Handlebars helper semantics).

lib/handlebars/compiler/compiler.js

@@ -408,6 +408,12 @@ Handlebars.JavaScriptCompiler = function() {};
     preamble: function() {
       var out = [];
 
+      // this register will disambiguate helper lookup from finding a function in
+      // a context. This is necessary for mustache compatibility, which requires
+      // that context functions in blocks are evaluated by blockHelperMissing, and
+      // then proceed as if the resulting value was provided to blockHelperMissing.
+      this.useRegister('foundHelper');
+
       if (!this.isChild) {
         var namespace = this.namespace;
         var copies = "helpers = helpers || " + namespace + ".helpers;";
@@ -520,10 +526,8 @@ Handlebars.JavaScriptCompiler = function() {};
         } else if (isScoped || this.options.knownHelpersOnly) {
           toPush = topStack + " = " + this.nameLookup('depth' + this.lastContext, name, 'context');
         } else {
-          toPush =  topStack + " = "
-              + this.nameLookup('helpers', name, 'helper')
-              + " || "
-              + this.nameLookup('depth' + this.lastContext, name, 'context');
+          this.register('foundHelper', this.nameLookup('helpers', name, 'helper'));
+          toPush = topStack + " = foundHelper || " + this.nameLookup('depth' + this.lastContext, name, 'context');
         }
 
         toPush += ';';
@@ -618,10 +622,10 @@ Handlebars.JavaScriptCompiler = function() {};
 
       params.push(stringOptions);
 
-      this.populateCall(params, id, helperId || id, fn);
+      this.populateCall(params, id, helperId || id, fn, program !== '{}');
     },
 
-    populateCall: function(params, id, helperId, fn) {
+    populateCall: function(params, id, helperId, fn, program) {
       var paramString = ["depth0"].concat(params).join(", ");
       var helperMissingString = ["depth0"].concat(helperId).concat(params).join(", ");
 
@@ -631,7 +635,8 @@ Handlebars.JavaScriptCompiler = function() {};
         this.source.push(nextStack + " = " + id + ".call(" + paramString + ");");
       } else {
         this.context.aliases.functionType = '"function"';
-        this.source.push("if(typeof " + id + " === functionType) { " + nextStack + " = " + id + ".call(" + paramString + "); }");
+        var condition = program ? "foundHelper && " : ""
+        this.source.push("if(" + condition + "typeof " + id + " === functionType) { " + nextStack + " = " + id + ".call(" + paramString + "); }");
       }
       fn.call(this, nextStack, helperMissingString, id);
       this.usingKnownHelper = false;