diff -u b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
--- b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -213,147 +213,281 @@
    */
   public function providerTestPostProcessExtension() {
     $data = [];
-    $data['manual_base_hook'] = [
-      'defined_functions' => ['cat', 'mouse', 'cheese'],
+
+    // This is test data for unit testing
+    // \Drupal\Core\Theme\Registry::postProcessExtension(), not what happens
+    // before it. Therefore, for all test data:
+    // - Explicitly defined hooks also come with explicitly defined preprocess
+    //   functions, because those are collected in
+    //   \Drupal\Core\Theme\Registry::processExtension().
+    // - Explicitly defined hooks that set a 'base hook' also have
+    //   'incomplete preprocess functions' set to TRUE, since that is done in
+    //   \Drupal\Core\Theme\Registry::processExtension().
+
+    // Test the discovery of suggestions via the presence of preprocess
+    // functions that follow the "__" naming pattern.
+    $data['base_hook_with_autodiscovered_suggestions'] = [
+      'defined_functions' => [
+        'test_preprocess_test_hook__suggestion',
+        'test_preprocess_test_hook__suggestion__another',
+      ],
       'hooks' => [
         'test_hook' => [
-          'preprocess functions' => ['cat', 'mouse'],
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
         ],
-        'test_hook_manual' => [
-          'preprocess functions' => ['cheese'],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'test_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+          ],
           'base hook' => 'test_hook',
-          'incomplete preprocess functions' => TRUE,
         ],
-        'test_hook_manual__foo' => [
-          'preprocess functions' => ['foo'],
-          'base hook' => 'test_hook_manual',
-          'incomplete preprocess functions' => TRUE,
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+            'test_preprocess_test_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    // Test that suggestions following the "__" naming pattern can also be
+    // explicitly defined in hook_theme(), such as 'field__node__title' defined
+    // in node_theme().
+    $data['base_hook_with_explicit_suggestions'] = [
+      'defined_functions' => [
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
         ],
-        'test_hook_manual__foo__bar' => [
-          'preprocess functions' => ['bar'],
-          'base hook' => 'test_hook_manual',
+        'test_hook__suggestion__another' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook__suggestion__another'],
           'incomplete preprocess functions' => TRUE,
         ],
       ],
       'expected' => [
         'test_hook' => [
-          'preprocess functions' => ['cat', 'mouse'],
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
         ],
-        'test_hook_manual' => [
-          'preprocess functions' => ['cat', 'mouse', 'cheese'],
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_test_hook__suggestion__another',
+          ],
           'base hook' => 'test_hook',
         ],
-        'test_hook_manual__foo' => [
-          'preprocess functions' => ['cat', 'mouse', 'cheese', 'foo'],
-          'base hook' => 'test_hook_manual',
+      ],
+    ];
+
+    // Same as above, but also test that a preprocess function for an
+    // intermediary suggestion level gets discovered.
+    $data['base_hook_with_explicit_suggestions_and_intermediary_preprocess_function'] = [
+      'defined_functions' => [
+        'test_preprocess_test_hook__suggestion'
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
         ],
-        'test_hook_manual__foo__bar' => [
-          'preprocess functions' => ['cat', 'mouse', 'cheese', 'foo', 'bar'],
-          'base hook' => 'test_hook_manual',
+        'test_hook__suggestion__another' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook__suggestion__another'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'test_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+            'explicit_preprocess_test_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
         ],
       ],
     ];
-    $data['missing_base_hook'] = [
-      'defined_functions' => ['cat', 'mouse', 'cheese'],
+
+    // Test that hooks not following the "__" naming pattern can explicitly
+    // specify a base hook, such as is done in
+    // \Drupal\Core\Layout\LayoutPluginManager::getThemeImplementations().
+    $data['child_hook_without_magic_naming'] = [
+      'defined_functions' => [
+      ],
       'hooks' => [
         'test_hook' => [
-          'preprocess functions' => ['cat', 'mouse'],
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
         ],
-        'test_hook_manual' => [
-          'preprocess functions' => ['cheese'],
-          'base hook' => 'missing_hook',
+        'child_hook' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_child_hook'],
           'incomplete preprocess functions' => TRUE,
         ],
       ],
       'expected' => [
         'test_hook' => [
-          'preprocess functions' => ['cat', 'mouse'],
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
         ],
-        'test_hook_manual' => [
-          'preprocess functions' => ['cheese'],
-          'base hook' => 'missing_hook',
+        'child_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+          ],
+          'base hook' => 'test_hook',
         ],
       ],
     ];
-    $data['manual_suggestion'] = [
+
+    // Same as above, but also test that such child hooks can also be extended
+    // with magically named suggestions.
+    /*
+    $data['child_hook_with_suggestions'] = [
       'defined_functions' => [
-        'cat',
-        'mouse',
-        'cheese',
-        'bread',
-        'test_preprocess_test_hook__suggestion',
-        'test_preprocess_test_hook__suggestion__another',
-        'and_now_for_something_completely_different',
+        'test_preprocess_child_hook__suggestion',
+        'test_preprocess_child_hook__suggestion__another',
       ],
       'hooks' => [
-        'unrelated_hook' => [
-          'preprocess functions' => ['and_now_for_something_completely_different'],
-        ],
         'test_hook' => [
-          'preprocess functions' => ['cat', 'mouse'],
-          'template' => 'test-hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
         ],
-        'test_hook__suggestion' => [
-          'preprocess functions' => ['bread'],
+        'child_hook' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_child_hook'],
           'incomplete preprocess functions' => TRUE,
         ],
-        'test_hook__suggestion__another' => [
-          'preprocess functions' => ['cheese'],
-          'incomplete preprocess functions' => TRUE,
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'child_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'child_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+            'test_preprocess_child_hook__suggestion',
+          ],
+          'base hook' => 'test_hook',
         ],
-        'test_hook__suggestion__with_base_hook' => [
-          'base hook' => 'unrelated_hook',
-          'preprocess functions' => ['something_new'],
+        'child_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+            'test_preprocess_child_hook__suggestion',
+            'test_preprocess_child_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+    */
+
+    // Test that a suggestion following the "__" naming pattern can specify a
+    // different base hook than what is implied by that pattern. Ensure that
+    // preprocess functions from both the naming pattern and from 'base hook'
+    // are collected.
+    $data['suggestion_with_alternate_base_hook'] = [
+      'defined_functions' => [
+        'test_preprocess_test_hook__suggestion'
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+        'alternate_base_hook' => [
+          'preprocess functions' => ['explicit_preprocess_alternate_base_hook'],
+        ],
+        'test_hook__suggestion__another' => [
+          'base hook' => 'alternate_base_hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook__suggestion__another'],
           'incomplete preprocess functions' => TRUE,
         ],
       ],
       'expected' => [
-        'unrelated_hook' => [
-          'preprocess functions' => ['and_now_for_something_completely_different'],
-        ],
         'test_hook' => [
           'preprocess functions' => [
-            'cat',
-            'mouse',
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'alternate_base_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_alternate_base_hook',
           ],
-          'template' => 'test-hook',
         ],
         'test_hook__suggestion' => [
           'preprocess functions' => [
-            'cat',
-            'mouse',
-            'bread',
+            'explicit_preprocess_test_hook',
             'test_preprocess_test_hook__suggestion',
           ],
           'base hook' => 'test_hook',
-          'template' => 'test-hook',
         ],
         'test_hook__suggestion__another' => [
           'preprocess functions' => [
-            'cat',
-            'mouse',
-            'bread',
+            'explicit_preprocess_alternate_base_hook',
+            'explicit_preprocess_test_hook',
             'test_preprocess_test_hook__suggestion',
-            'cheese',
-            'test_preprocess_test_hook__suggestion__another',
+            'explicit_preprocess_test_hook__suggestion__another',
           ],
+          'base hook' => 'alternate_base_hook',
+        ],
+      ],
+    ];
+
+    // Test when a base hook is missing.
+    $data['missing_base_hook'] = [
+      'defined_functions' => [
+      ],
+      'hooks' => [
+        'child_hook' => [
           'base hook' => 'test_hook',
-          'template' => 'test-hook',
+          'preprocess functions' => ['explicit_preprocess_child_hook'],
+          'incomplete preprocess functions' => TRUE,
         ],
-        'test_hook__suggestion__with_base_hook' => [
+      ],
+      'expected' => [
+        'child_hook' => [
           'preprocess functions' => [
-            'and_now_for_something_completely_different',
-            'cat',
-            'mouse',
-            'bread',
-            'test_preprocess_test_hook__suggestion',
-            'something_new',
+            'explicit_preprocess_child_hook',
           ],
-          'base hook' => 'unrelated_hook',
-          'template' => 'test-hook',
+          'base hook' => 'test_hook',
         ],
       ],
     ];
+
     return $data;
   }