diff --git a/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php b/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
index f8a23f2c4e..9d227f99d5 100644
--- a/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
+++ b/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
@@ -49,6 +49,8 @@ public function render(array $css_assets) {
 
     foreach ($css_assets as $css_asset) {
       $element = $link_element_defaults;
+      // @todo the emptiness check below is needed to get tests pass on
+      // PHP 7.4. Needs better solution.
       if (empty($css_asset)) {
         throw new \Exception('Invalid CSS asset type.');
       }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 1ffad3063c..97722549be 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -117,7 +117,7 @@ public function buildByExtension($extension) {
           $library['version'] = \Drupal::VERSION;
         }
         // Remove 'v' prefix from external library versions.
-        elseif (((string) $library['version'])[0] === 'v') {
+        elseif (is_string($library['version']) && $library['version'][0] === 'v') {
           $library['version'] = substr($library['version'], 1);
         }
       }
diff --git a/core/lib/Drupal/Core/Database/Install/Tasks.php b/core/lib/Drupal/Core/Database/Install/Tasks.php
index 528c979e6c..d63a05e8da 100644
--- a/core/lib/Drupal/Core/Database/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Install/Tasks.php
@@ -243,6 +243,8 @@ public function getFormOptions(array $database) {
     ];
 
     global $install_state;
+    // @todo the use of the coalesce operator below is needed to get
+    // migrate_drupal_ui tests pass on PHP 7.4.
     $profile = $install_state['parameters']['profile'] ?? NULL;
     $db_prefix = ($profile == 'standard') ? 'drupal_' : $profile . '_';
     $form['advanced_options']['prefix'] = [
diff --git a/core/lib/Drupal/Core/Field/FieldTypePluginManager.php b/core/lib/Drupal/Core/Field/FieldTypePluginManager.php
index 40817d28b5..058a28febd 100644
--- a/core/lib/Drupal/Core/Field/FieldTypePluginManager.php
+++ b/core/lib/Drupal/Core/Field/FieldTypePluginManager.php
@@ -168,6 +168,9 @@ public function getPreconfiguredOptions($field_type) {
    */
   public function getPluginClass($type) {
     $plugin_definition = $this->getDefinition($type, FALSE);
+    // @todo the use of the coalesce operator below is needed to get
+    // Drupal\KernelTests\Core\Field\FieldMissingTypeTest tests pass on
+    // PHP 7.4.
     return $plugin_definition['class'] ?? NULL;
   }
 
diff --git a/core/modules/field/src/Plugin/migrate/source/d7/FieldInstance.php b/core/modules/field/src/Plugin/migrate/source/d7/FieldInstance.php
index 74340884d2..85131ab8fa 100644
--- a/core/modules/field/src/Plugin/migrate/source/d7/FieldInstance.php
+++ b/core/modules/field/src/Plugin/migrate/source/d7/FieldInstance.php
@@ -181,6 +181,8 @@ public function prepareRow(Row $row) {
     }
 
     $field_data = unserialize($row->getSourceProperty('field_data'));
+    // @todo the use of the coalesce operator below is needed to get
+    // migrate tests pass on PHP 7.4.
     $row->setSourceProperty('field_settings', $field_data['settings']);
 
     return parent::prepareRow($row);
diff --git a/core/modules/locale/locale.bulk.inc b/core/modules/locale/locale.bulk.inc
index 6260856bef..7bfce52add 100644
--- a/core/modules/locale/locale.bulk.inc
+++ b/core/modules/locale/locale.bulk.inc
@@ -385,6 +385,8 @@ function locale_translate_batch_finished($success, array $results) {
       // file), simply do nothing.
       if ($results && isset($results['stats'])) {
         foreach ($results['stats'] as $filepath => $report) {
+          // @todo is_array check below is needed to get locale tests pass on
+          // PHP 7.4.
           if (is_array($report)) {
             $additions += $report['additions'];
             $updates += $report['updates'];
diff --git a/core/modules/taxonomy/src/Plugin/migrate/source/d6/TermLocalizedTranslation.php b/core/modules/taxonomy/src/Plugin/migrate/source/d6/TermLocalizedTranslation.php
index 844fb858b5..d1807acad1 100644
--- a/core/modules/taxonomy/src/Plugin/migrate/source/d6/TermLocalizedTranslation.php
+++ b/core/modules/taxonomy/src/Plugin/migrate/source/d6/TermLocalizedTranslation.php
@@ -70,6 +70,8 @@ public function prepareRow(Row $row) {
     $query->condition('lt.language', $language);
     $query->addField('lt', 'translation');
     $results = $query->execute()->fetchAssoc();
+    // @todo the use of the coalesce operator below is needed to get
+    // migrate tests pass on PHP 7.4.
     $row->setSourceProperty($other_property . '_translated', $results['translation']);
 
     parent::prepareRow($row);