diff --git a/NEWS b/NEWS
index 49b61d236967..fea16290736c 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,10 @@ PHP                                                                        NEWS
     IntlCalendar::equals(), ::before(), ::after(), and ::isEquivalentTo().
     (Weilin Du)
 
+- Opcache:
+  . Fixed bug GH-20469 (Crash during inheritance cache lookup with nested
+    autoloading). (Levi Morrison)
+
 - Phar:
   . Fixed a bypass of the magic ".phar" directory protection in
     Phar::addEmptyDir() for paths starting with "/.phar", while allowing
diff --git a/Zend/zend_inheritance.c b/Zend/zend_inheritance.c
index eba21dd8e82a..8846633559d7 100644
--- a/Zend/zend_inheritance.c
+++ b/Zend/zend_inheritance.c
@@ -310,14 +310,23 @@ static zend_class_entry *lookup_class(zend_class_entry *scope, zend_string *name
 
 /* Instanceof that's safe to use on unlinked classes. */
 static bool unlinked_instanceof(zend_class_entry *ce1, const zend_class_entry *ce2) {
+	/* Do not short-circuit to instanceof_function() here:
+	 *
+	 * 	if (ce1->ce_flags & ZEND_ACC_LINKED) {
+	 * 		return instanceof_function(ce1, ce2);
+	 * 	}
+	 *
+	 * See ext/opcache/tests/gh20469.phpt. During inheritance cache lookups,
+	 * autoloading may re-enter class linking in a way where ce1 itself is
+	 * linked, but a class in its parent chain is still unlinked. The normal
+	 * instanceof_function() assumes the whole parent chain is linked, and may
+	 * interpret an unresolved parent_name as a zend_class_entry (and crash).
+	 */
+
 	if (ce1 == ce2) {
 		return 1;
 	}
 
-	if (ce1->ce_flags & ZEND_ACC_LINKED) {
-		return instanceof_function(ce1, ce2);
-	}
-
 	if (ce1->parent) {
 		zend_class_entry *parent_ce;
 		if (ce1->ce_flags & ZEND_ACC_RESOLVED_PARENT) {
diff --git a/ext/opcache/tests/gh20469.phpt b/ext/opcache/tests/gh20469.phpt
new file mode 100644
index 000000000000..1cd826c177ef
--- /dev/null
+++ b/ext/opcache/tests/gh20469.phpt
@@ -0,0 +1,134 @@
+--TEST--
+GH-20469: Inheritance cache with reentrant autoloading must not crash
+--EXTENSIONS--
+opcache
+--CONFLICTS--
+server
+--FILE--
+<?php
+$dir = __DIR__ . '/gh20469';
+@mkdir($dir . '/classes', 0777, true);
+
+file_put_contents($dir . '/autoload.php', <<<'PHP'
+<?php
+spl_autoload_register(function ($class) {
+    $prefix = 'APP\\';
+    if (strncmp($class, $prefix, strlen($prefix)) === 0) {
+        require __DIR__ . '/classes/' . substr($class, strlen($prefix)) . '.php';
+    }
+});
+PHP);
+
+/* The dependency cycle is:
+ * ChildOfParentBeingLinked -> ParentBeingLinked -> CovariantReturnWithTrait
+ * -> RequiresRootReturnTrait -> ChildOfParentBeingLinked.
+ */
+file_put_contents($dir . '/test1.php', <<<'PHP'
+<?php
+require __DIR__ . '/autoload.php';
+echo \APP\ChildOfParentBeingLinked::SOME_CONSTANT;
+PHP);
+
+file_put_contents($dir . '/test2.php', <<<'PHP'
+<?php
+require __DIR__ . '/autoload.php';
+echo \APP\ParentBeingLinked::SOME_CONSTANT;
+PHP);
+
+file_put_contents($dir . '/classes/RootForTraitReturn.php', <<<'PHP'
+<?php
+namespace APP;
+
+class RootForTraitReturn
+{
+    function createResult(): BaseCovariantReturn
+    {
+    }
+}
+PHP);
+
+file_put_contents($dir . '/classes/ParentBeingLinked.php', <<<'PHP'
+<?php
+namespace APP;
+
+class ParentBeingLinked extends RootForTraitReturn
+{
+    public const SOME_CONSTANT = 3;
+
+    function createResult(): CovariantReturnWithTrait
+    {
+    }
+}
+PHP);
+
+file_put_contents($dir . '/classes/ChildOfParentBeingLinked.php', <<<'PHP'
+<?php
+namespace APP;
+
+class ChildOfParentBeingLinked extends ParentBeingLinked
+{
+}
+PHP);
+
+file_put_contents($dir . '/classes/BaseCovariantReturn.php', <<<'PHP'
+<?php
+namespace APP;
+
+abstract class BaseCovariantReturn
+{
+}
+PHP);
+
+file_put_contents($dir . '/classes/RequiresRootReturnTrait.php', <<<'PHP'
+<?php
+namespace APP;
+
+trait RequiresRootReturnTrait
+{
+    abstract function build(): RootForTraitReturn;
+}
+PHP);
+
+file_put_contents($dir . '/classes/CovariantReturnWithTrait.php', <<<'PHP'
+<?php
+namespace APP;
+
+class CovariantReturnWithTrait extends BaseCovariantReturn
+{
+    use RequiresRootReturnTrait;
+
+    function build(): ChildOfParentBeingLinked
+    {
+    }
+}
+PHP);
+
+include 'php_cli_server.inc';
+$ini = trim((string) getenv('TEST_PHP_EXTRA_ARGS'));
+$ini .= ($ini !== '' ? ' ' : '') . '-d opcache.enable=1 -d opcache.enable_cli=1 -d opcache.file_update_protection=0';
+php_cli_server_start($ini);
+
+echo file_get_contents('http://' . PHP_CLI_SERVER_ADDRESS . '/gh20469/test1.php'), "\n";
+echo file_get_contents('http://' . PHP_CLI_SERVER_ADDRESS . '/gh20469/test2.php'), "\n";
+?>
+--CLEAN--
+<?php
+$dir = __DIR__ . '/gh20469';
+if (is_dir($dir)) {
+    $iterator = new RecursiveIteratorIterator(
+        new RecursiveDirectoryIterator($dir, RecursiveDirectoryIterator::SKIP_DOTS),
+        RecursiveIteratorIterator::CHILD_FIRST
+    );
+    foreach ($iterator as $file) {
+        if ($file->isDir()) {
+            rmdir($file->getPathname());
+        } else {
+            unlink($file->getPathname());
+        }
+    }
+    rmdir($dir);
+}
+?>
+--EXPECT--
+3
+3