why can not find required controller?

index.coffee

    
module.directive('foo', [
  '$log', function($log) {
    return {
      restrict: 'E',
      replace: true,
      transclude: true,
      template: '<div id="test" ng-transclude></div>',
      controller: function($scope) {
        this.scope = $scope;
        return $scope.panes = [];
      },
      link: function(scope, element, attrs) {
        return $log.log('test1', scope.panes);
      }
    };
  }
]);

module.directive('bar', [
  '$log', function($log) {
    return {
      restrict: 'E',
      replace: true,
      transclude: true,
      require: '^?foo',
      controller: function($scope) {
        return this.x = 1;
      },
      template: '<div ng-transclude></div>',
      link: function(scope, element, attrs, fooCtrl) {
        return $log.log('test2', fooCtrl);
      }
    };
  }
]);

// below is the piece of html:
<foo ng-controller="IndexController">
      <bar></bar>
    </foo>
// below is the element generated, inspected from chrome developer tools
<div id="test" ng-transclude="" ng-controller="IndexController" class="ng-scope">
      <div ng-transclude="" class="ng-scope"></div>
    </div>
    
// below is console output:
test2 
Array[0]
length: 0
__proto__: Array[0]
 angular.js:5930
test1 
Array[0]
length: 0
__proto__: Array[0]  
  

帮你提交到这, 好分析点: http://plnkr.co/edit/aM81utpVPwYoI6ru28rT?p=preview
不过没看懂你想做什么。
this.scope = $scope; 和 return this.x = 1; 这两句代码干嘛的? 为啥要用this