Core: Ensure wp_using_ext_object_cache() always returns a boolean

src/wp-includes/load.php

@@ -810,13 +810,19 @@ function wp_set_wpdb_vars() {
 function wp_using_ext_object_cache( $using = null ) {
 	global $_wp_using_ext_object_cache;
 
+	// Save the current state to return later.
 	$current_using = $_wp_using_ext_object_cache;
 
 	if ( null !== $using ) {
-		$_wp_using_ext_object_cache = $using;
+		$_wp_using_ext_object_cache = (bool) $using;
 	}
 
-	return $current_using;
+	if ( null === $_wp_using_ext_object_cache ) {
+		// If the global is uninitialized, the value would be null, which violates the type signature.
+		return false;
+	}
+
+	return (bool) $current_using;
 }
 
 /**

tests/phpunit/tests/load/wpUsingExtObjectCache.php

@@ -0,0 +1,32 @@
+<?php
+
+/**
+ * Tests for wp_using_ext_object_cache().
+ *
+ * @group load
+ *
+ * @covers ::wp_using_ext_object_cache
+ */
+class Tests_Load_wpUsingExtObjectCache extends WP_UnitTestCase {
+
+	private $orig_using_ext_cache;
+
+	public function set_up() {
+		parent::set_up();
+		global $_wp_using_ext_object_cache;
+
+		$this->orig_using_ext_cache = $_wp_using_ext_object_cache;
+	}
+
+	public function tear_down() {
+		global $_wp_using_ext_object_cache;
+
+		$_wp_using_ext_object_cache = $this->orig_using_ext_cache;
+		parent::tear_down();
+	}
+
+	public function test_should_always_return_boolean() {
+		wp_using_ext_object_cache( 1 );
+		$this->assertIsBool( wp_using_ext_object_cache() );
+	}
+}