Ticket #39295: 39295.2.diff

src/wp-includes/l10n.php

@@ -101 +101 @@
                 $user = get_user_by( 'id', $user_id );
         }
 
-        if ( ! $user ) {
-                return get_locale();
+        if ( $user && $user->locale ) {
+                $locale = $user->locale;
+        } elseif ( doing_filter( 'locale' ) ) {
+                _doing_it_wrong(
+                        __FUNCTION__,
+                        /* translators: %s: 'locale' filter name. */
+                        sprintf( __( 'It cannot be used on the %s filter.' ), '<code>locale</code>' ),
+                        '5.6.0'
+                );
+        } else {
+                $locale = get_locale();
         }
 
-        $locale = $user->locale;
-        return $locale ? $locale : get_locale();
+        if ( empty( $locale ) ) {
+                $locale = 'en_US';
+        }
+
+        return $locale;
 }
 
 /**

tests/phpunit/tests/l10n/getUserLocale.php

@@ -136 +136 @@
                 $user_locale = get_user_locale( 'string' );
                 $this->assertSame( get_locale(), $user_locale );
         }
+
+        /**
+         * @ticket 39295
+         */
+        public function test_get_user_locale_as_locale_filter_triggers_doing_it_wrong() {
+                wp_set_current_user( 0 );
+                switch_to_locale( 'de_DE');
+
+                $user_locale1 = get_user_locale();
+
+                add_filter( 'locale', 'get_user_locale' );
+
+                $this->setExpectedIncorrectUsage( 'get_user_locale' );
+                $user_locale2 = get_user_locale();
+
+                remove_filter( 'locale', 'get_user_locale' );
+
+                restore_current_locale();
+
+                $this->assertSame( 'de_DE', $user_locale1 );
+                $this->assertSame( 'en_US', $user_locale2 );
+        }
 }