diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index cb7d9e7..1732b86 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -61,51 +61,15 @@ public function __construct(CacheCollectorInterface $library_discovery_collector
    * {@inheritdoc}
    */
   public function getLibrariesByExtension($extension) {
-    if (!isset($this->libraryDefinitions[$extension])) {
-      $libraries = $this->collector->get($extension);
-      $this->libraryDefinitions[$extension] = [];
-      foreach ($libraries as $name => $definition) {
-        $library_name = "$extension/$name";
-        $this->libraryDefinitions[$extension][$name] = $definition;
-      }
-    }
-
-    return $this->libraryDefinitions[$extension];
+    return $this->getLibraryDefinitions($extension);
   }
 
   /**
    * {@inheritdoc}
    */
   public function getLibraryByName($extension, $name) {
-    $extension = $this->getLibrariesByExtension($extension);
-    if (isset($extension[$name])) {
-      // Handle libraries that are marked for override or removal.
-      // @see \Drupal\Core\Asset\LibraryDiscoveryParser::applyLibrariesOverride()
-      if (isset($extension[$name]['override'])) {
-        $override = $extension[$name]['override'];
-        if ($override === FALSE) {
-          // Remove the library definition if FALSE is given.
-          unset($extension[$name]);
-          return FALSE;
-        }
-        else {
-          // Otherwise replace with existing library definition if it exists.
-          // Throw an exception if it doesn't.
-          list($new_extension, $new_name) = explode('/', $override);
-          if ($library = $this->getLibraryByName($new_extension, $new_name)) {
-            $extension[$name] = $library;
-          }
-          else {
-            throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $override));
-          }
-        }
-      }
-      return $extension[$name];
-    }
-    else {
-      return FALSE;
-    }
-
+    $extension = $this->getLibraryDefinitions($extension);
+    return isset($extension[$name]) ? $extension[$name] : FALSE;
   }
 
   /**
@@ -113,6 +77,39 @@ public function getLibraryByName($extension, $name) {
    */
   public function clearCachedDefinitions() {
     $this->cacheTagInvalidator->invalidateTags(['library_info']);
+    $this->libraryDefinitions = [];
+    $this->collector->clear();
+  }
+
+  protected function getLibraryDefinitions($extension) {
+    if (!isset($this->libraryDefinitions[$extension])) {
+      $libraries = $this->collector->get($extension);
+      $this->libraryDefinitions[$extension] = [];
+      foreach ($libraries as $name => $definition) {
+        // Handle libraries that are marked for override or removal.
+        // @see \Drupal\Core\Asset\LibraryDiscoveryParser::applyLibrariesOverride()
+        if (isset($definition['override'])) {
+          if ($definition['override'] === FALSE) {
+            // Remove the library definition if FALSE is given.
+            unset($libraries[$name]);
+          }
+          else {
+            // Otherwise replace with existing library definition if it exists.
+            // Throw an exception if it doesn't.
+            list($replacement_extension, $replacement_name) = explode('/', $definition['override']);
+            $replacement_definition = $this->getLibraryDefinitions($replacement_extension);
+            if (isset($replacement_definition[$replacement_name])) {
+              $libraries[$name] = $replacement_definition[$replacement_name];
+            }
+            else {
+              throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $definition['override']));
+            }
+          }
+        }
+      }
+      $this->libraryDefinitions[$extension] = $libraries;
+    }
+    return $this->libraryDefinitions[$extension];
   }
 
 }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 7791869..427640f 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -426,14 +426,16 @@ protected function setOverrideValue(array &$library, array $sub_key, array $over
       // not exist, then throw an exception.
       $key_exists = NULL;
       $parents = array_merge($sub_key, [$original]);
-      // Ensure the replacement path is relative to drupal root.
-      $replacement = $this->resolveThemeAssetPath($theme_path, $replacement);
-      $new_parents = array_merge($sub_key, [$replacement]);
+      // Save the attributes of the library asset to be overridden.
       $attributes = NestedArray::getValue($library, $parents, $key_exists);
       if ($key_exists) {
-        // Remove asset to be overridden, but keep its attributes.
+        // Remove asset to be overridden.
         NestedArray::unsetValue($library, $parents);
+        // No need to replace if FALSE is specified, since that is a removal.
         if ($replacement) {
+          // Ensure the replacement path is relative to drupal root.
+          $replacement = $this->resolveThemeAssetPath($theme_path, $replacement);
+          $new_parents = array_merge($sub_key, [$replacement]);
           // Replace with an override if specified.
           NestedArray::setValue($library, $new_parents, $attributes);
         }
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
index d82f1d8..52bb147 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
@@ -3,10 +3,11 @@ global-styling:
   css:
     base:
       kitten.css: {}
+
 collapse:
+  version: VERSION
   js:
     js/collapse.js: { }
-
   css:
     base:
       css/collapse.css: { }