New Twiddle

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  enabled: false,
  items: ['a'],
  isNotEmpty: Ember.computed.notEmpty('items'),
  
  actions: {
    addItem: function() {
      this.get('items').pushObject('another');
    },
    removeItem: function() {
      this.get('items').removeAt(0);
    }
  }
});

application.template.hbs

<h1>inline if within concat bug</h1>
<h2 style="color: green">press the remove button</h2>

<button {{action 'addItem'}}>Add</button>
<button {{action 'removeItem'}}>Remove</button> items: {{items.length}}

<hr />
Has Items: [{{concat (if items 'T' 'F')}}] (array in concat)<br />
Has Items: [{{if items 'T' 'F'}}] (array simple)

<hr />
Has Items: [{{concat (if isNotEmpty 'T' 'F')}}] (boolean in concat)<br />
Has Items: [{{if isNotEmpty 'T' 'F'}}] (boolean simple)

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}