diff -u b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
--- b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
@@ -3,7 +3,6 @@
 namespace Drupal\Component\Plugin\Discovery;
 
 use Drupal\Component\Plugin\Definition\DerivablePluginDefinitionInterface;
-use Drupal\Component\Plugin\Definition\PluginDefinitionInterface;
 use Drupal\Component\Plugin\Exception\InvalidDeriverException;
 
 /**
@@ -56,7 +55,7 @@
     // $base_plugin_id.
     $plugin_definition = $this->decorated->getDefinition($plugin_id, FALSE);
 
-    [$base_plugin_id, $derivative_id] = $this->decodePluginId($plugin_id);
+    list($base_plugin_id, $derivative_id) = $this->decodePluginId($plugin_id);
     $base_plugin_definition = $this->decorated->getDefinition($base_plugin_id, $exception_on_invalid);
     if ($base_plugin_definition) {
       $deriver = $this->getDeriver($base_plugin_id, $base_plugin_definition);
@@ -72,7 +71,7 @@
         }
         // It is vital that derivative plugin definitions contain derivative
         // plugin IDs. Enforce it here, because not all derivers do it.
-        $this->setPluginIdsOnDefinition($plugin_definition, $base_plugin_id, $plugin_id);
+        $this->setPluginIdOnDefinition($plugin_definition, $plugin_id, $base_plugin_id);
       }
     }
 
@@ -112,7 +111,7 @@
           }
           // It is vital that derivative plugin definitions contain derivative
           // plugin IDs. Enforce it here, because not all derivers do it.
-          $this->setPluginIdsOnDefinition($derivative_plugin_definition, $base_plugin_id, $plugin_id);
+          $this->setPluginIdOnDefinition($derivative_plugin_definition, $plugin_id, $base_plugin_id);
           $plugin_definitions[$plugin_id] = $derivative_plugin_definition;
         }
       }
@@ -234,47 +233,37 @@
   /**
    * Merges a base and derivative definition, taking into account empty values.
    *
-   * @param mixed[]|\Drupal\Component\Plugin\Definition\PluginDefinitionInterface $base_plugin_definition
+   * @param array $base_plugin_definition
    *   The base plugin definition.
-   * @param mixed[]|\Drupal\Component\Plugin\Definition\MergeablePluginDefinitionInterface $derivative_definition
+   * @param array $derivative_definition
    *   The derivative plugin definition.
    *
-   * @return mixed[]|\Drupal\Component\Plugin\Definition\PluginDefinitionInterface
+   * @return array
    *   The merged definition.
    */
   protected function mergeDerivativeDefinition($base_plugin_definition, $derivative_definition) {
-    if (is_array($base_plugin_definition) && is_array($derivative_definition)) {
-      // Use this definition as defaults if a plugin already defined itself as
-      // this derivative, but filter out empty values first.
-      $filtered_base = array_filter($base_plugin_definition);
-      $derivative_definition = $filtered_base + ($derivative_definition ?: []);
-      // Add back any empty keys that the derivative didn't have.
-      return $derivative_definition + $base_plugin_definition;
-    }
-    elseif ($derivative_definition instanceof MergeablePluginDefinitionInterface && $base_plugin_definition instanceof PluginDefinitionInterface) {
-      return $derivative_definition->mergeDefaultDefinition($base_plugin_definition);
-    }
-    else {
-      $base_plugin_definition_type = is_object($base_plugin_definition) ? get_class($base_plugin_definition) : gettype($base_plugin_definition);
-      $derivative_plugin_definition_type = is_object($derivative_definition) ? get_class($derivative_definition) : gettype($derivative_definition);
-      throw new \InvalidArgumentException(sprintf('Could not merge base plugin definitions of type %s into derivative plugin definition of type %s. Both must be arrays, or the derivative plugin definition must implement %s and the base plugin definition must implement %s.', $base_plugin_definition_type, $derivative_plugin_definition_type, MergeablePluginDefinitionInterface::class, PluginDefinitionInterface::class));
-    }
+    // Use this definition as defaults if a plugin already defined itself as
+    // this derivative, but filter out empty values first.
+    $filtered_base = array_filter($base_plugin_definition);
+    $derivative_definition = $filtered_base + ($derivative_definition ?: []);
+    // Add back any empty keys that the derivative didn't have.
+    return $derivative_definition + $base_plugin_definition;
   }
 
   /**
    * Sets the plugin ID on a plugin definition.
    *
-   * @param string[] $plugin_definition
+   * @param mixed[]|\Drupal\Component\Plugin\Definition\PluginDefinitionInterface $plugin_definition
    *   The plugin definition.
-   * @param string $base_plugin_id
-   *   The base plugin id.
    * @param string $plugin_id
    *   The plugin ID to set.
+   * @param string $base_plugin_id
+   *   The base plugin ID to set.
    */
-  protected function setPluginIdsOnDefinition(array &$plugin_definition, $base_plugin_id, $plugin_id) {
+  protected function setPluginIdOnDefinition(&$plugin_definition, $plugin_id, $base_plugin_id) {
     if (is_array($plugin_definition)) {
-      $plugin_definition['base_id'] = $base_plugin_id;
       $plugin_definition['id'] = $plugin_id;
+      $plugin_definition['base_id'] = $base_plugin_id;
     }
   }
 
diff -u b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
--- b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -981,7 +981,7 @@
       $migration_manager = $this->getMigrationPluginManager();
       $migrations = $migration_manager->getDefinitions();
       foreach ($migrations as $migration_id => $migration) {
-        if ($base_id === ($migration['base_id'] ?? NULL)) {
+        if ($base_id === substr($migration_id, 0, strpos($migration_id, $this::DERIVATIVE_SEPARATOR))) {
           // Get this derived migration's mapping table and add it to the list
           // of mapping tables to look in for the highest ID.
           $stub = $migration_manager->createInstance($migration_id);
diff -u b/core/tests/Drupal/Tests/Core/Plugin/Discovery/DerivativeDiscoveryDecoratorTest.php b/core/tests/Drupal/Tests/Core/Plugin/Discovery/DerivativeDiscoveryDecoratorTest.php
--- b/core/tests/Drupal/Tests/Core/Plugin/Discovery/DerivativeDiscoveryDecoratorTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/Discovery/DerivativeDiscoveryDecoratorTest.php
@@ -212,6 +212,7 @@
 
     $expected = $definitions['non_container_aware_discovery'];
     $expected['id'] = 'non_container_aware_discovery:test_discovery_1';
+    $expected['base_id'] = 'non_container_aware_discovery';
     $this->assertArrayEquals($expected, $returned_definitions['non_container_aware_discovery:test_discovery_1']);
   }
 
@@ -252,6 +253,7 @@
 
     $expected = $base_definition;
     $expected['id'] = 'non_container_aware_discovery:test_discovery_1';
+    $expected['base_id'] = 'non_container_aware_discovery';
     $this->assertArrayEquals($expected, $discovery->getDefinition('non_container_aware_discovery:test_discovery_1'));
   }
 
only in patch2:
unchanged:
--- a/core/tests/Drupal/KernelTests/Core/Plugin/DerivativeTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Plugin/DerivativeTest.php
@@ -19,7 +19,13 @@ class DerivativeTest extends PluginTestBase {
    */
   public function testDerivativeDecorator() {
     // Ensure that getDefinitions() returns the expected definitions.
-    $this->assertEqual($this->mockBlockManager->getDefinitions(), $this->mockBlockExpectedDefinitions);
+    $expected = $this->mockBlockExpectedDefinitions;
+    $expected['menu:main_menu']['base_id'] = 'menu';
+    $expected['menu:navigation']['base_id'] = 'menu';
+    $expected['menu:foo']['base_id'] = 'menu';
+    $expected['layout']['base_id'] = 'layout';
+    $expected['layout:foo']['base_id'] = 'layout';
+    $this->assertEquals($this->mockBlockManager->getDefinitions(), $expected);
 
     // Ensure that getDefinition() returns the expected definition.
     foreach ($this->mockBlockExpectedDefinitions as $id => $definition) {
only in patch2:
unchanged:
--- a/core/tests/Drupal/KernelTests/Core/Plugin/InspectionTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Plugin/InspectionTest.php
@@ -30,6 +30,9 @@ public function testInspection() {
     foreach (['user_login', 'layout'] as $id) {
       $plugin = $this->mockBlockManager->createInstance($id);
       $expected_definition = $this->mockBlockExpectedDefinitions[$id];
+      if ($id == 'layout') {
+        $expected_definition['base_id'] = 'layout';
+      }
       $this->assertIdentical($plugin->getPluginId(), $id);
       $this->assertIdentical($this->castSafeStrings($this->mockBlockManager->getDefinition($id)), $expected_definition);
       $this->assertIdentical($this->castSafeStrings($plugin->getPluginDefinition()), $expected_definition);