diff -u b/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
--- b/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -314,8 +314,9 @@
   public function rebuildForm($form_id, FormStateInterface &$form_state, $old_form = NULL) {
     $form = $this->retrieveForm($form_id, $form_state);
 
-    // We don't allow to set state on GET requests.
-    if (!$form_state->isMethodType('GET')) {
+    // Persisting $form_state is a side-effect disallowed during a "safe" HTTP
+    // method (http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1).
+    if ($this->requestStack->getCurrentRequest()->isMethodSafe()) {
       $form_state->setCached();
     }
 
diff -u b/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
--- b/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -475,9 +475,12 @@
    * {@inheritdoc}
    */
   public function setCached($cache = TRUE) {
-    if ($this->isMethodType('GET')) {
-      throw new \LogicException('Caching on GET requests is not allowed.');
+    // Persisting $form_state is a side-effect disallowed during a "safe" HTTP
+    // method (http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1).
+    if ($cache && \Drupal::request()->isMethodSafe()) {
+      throw new \LogicException(sprintf('Form state caching on %s requests is not allowed.', \Drupal::request()->getMethod()));
     }
+
     $this->cache = (bool) $cache;
     return $this;
   }
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -640,6 +640,10 @@ public function getButtons();
    *   TRUE if the form should be cached, FALSE otherwise.
    *
    * @return $this
+   *
+   * @throws \LogicException
+   *   If the current request is using an HTTP method that must not change
+   *   state (e.g., GET).
    */
   public function setCached($cache = TRUE);