diff -u b/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
--- b/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -137,8 +137,11 @@
    */
   public function getFunctions() {
     return [
-      // This function will receive a renderable array, if an array is detected.
+      // This function will receive a renderable array object, if detected.
       new \Twig_SimpleFunction('render_var', [$this, 'renderVar']),
+      // This function casts a variable to bool, with special conditions for
+      // empty \ReadOnlyArrayObject objects.
+      new \Twig_SimpleFunction('to_bool', [$this, 'toBool']),
       // The url and path function are defined in close parallel to those found
       // in \Symfony\Bridge\Twig\Extension\RoutingExtension
       new \Twig_SimpleFunction('url', [$this, 'getUrl'], ['is_safe_callback' => [$this, 'isUrlGenerationSafe']]),
@@ -641,2 +644,18 @@
 
+  /**
+   * Casts a variable to bool.
+   *
+   * @param mixed $arg
+   *   The variable that needs casted to a boolean.
+   *
+   * @return mixed
+   *   The rendered output or an Twig_Markup object.
+   */
+  public function toBool($arg) {
+    if ($arg instanceof ReadOnlyArrayObject) {
+      return count($arg) > 0;
+    }
+    return (bool) $arg;
+  }
+
 }
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Template/TwigNodeVisitor.php
+++ b/core/lib/Drupal/Core/Template/TwigNodeVisitor.php
@@ -50,6 +50,42 @@ protected function doLeaveNode(\Twig_Node $node, \Twig_Environment $env) {
         $this->skipRenderVarFunction = TRUE;
       }
     }
+    // For if statements checking a variable, wrap the variable in a custom
+    // boolean cast that casts empty ReadOnlyArrayObjects as true.
+    elseif ($node instanceof \Twig_Node_If) {
+      $tests = $node->getNode('tests');
+      if (!empty($tests)) {
+        foreach ($tests as $name => $test) {
+          // Covers if statements like {% if variable %}.
+          if ($test instanceof \Twig_Node_Expression_Name || $test instanceof \Twig_Node_Expression_GetAttr) {
+            $test = new \Twig_Node_Expression_Function('to_bool', new \Twig_Node([$test]), $node->getLine());
+          }
+          // Covers if statements like {% if not variable %}
+          else if ($test instanceof \Twig_Node_Expression_Unary) {
+            $subtest = $test->getNode('node');
+            if ($subtest instanceof \Twig_Node_Expression_Name || $subtest instanceof \Twig_Node_Expression_GetAttr) {
+              $wrapper = new \Twig_Node_Expression_Function('to_bool', new \Twig_Node([$subtest]), $node->getLine());
+              $test->setNode('node', $wrapper);
+            }
+          }
+          // Covers if statements like {% if variable or other_variable %}
+          else if ($test instanceof \Twig_Node_Expression_Binary_And || $test instanceof \Twig_Node_Expression_Binary_Or) {
+            $left = $test->getNode('left');
+            $right = $test->getNode('right');
+            if ($left instanceof \Twig_Node_Expression_Name || $left instanceof \Twig_Node_Expression_GetAttr) {
+              $wrapper = new \Twig_Node_Expression_Function('to_bool', new \Twig_Node([$left]), $node->getLine());
+              $test->setNode('left', $wrapper);
+            }
+            if ($right instanceof \Twig_Node_Expression_Name || $right instanceof \Twig_Node_Expression_GetAttr) {
+              $wrapper = new \Twig_Node_Expression_Function('to_bool', new \Twig_Node([$right]), $node->getLine());
+              $test->setNode('right', $wrapper);
+            }
+          }
+          $tests->setNode($name, $test);
+        }
+      }
+      $node->setNode('tests', $tests);
+    }
 
     return $node;
   }