Merge branch '3.4' into 4.3

* 3.4:
  #30432 fix an error message
  fix paths to detect code owners
  [Validator] Ensure numeric subpaths do not cause errors on PHP 7.4
  Remove unused local variables in tests
  Make sure to collect child forms created on *_SET_DATA events
  do not render errors for checkboxes twice

DebugClassLoader.php

@@ -482,7 +482,7 @@ class DebugClassLoader
         }
 
         if (isset($dirFiles[$file])) {
-            return $real .= $dirFiles[$file];
+            return $real.$dirFiles[$file];
         }
 
         $kFile = strtolower($file);
@@ -501,7 +501,7 @@ class DebugClassLoader
             self::$darwinCache[$kDir][1] = $dirFiles;
         }
 
-        return $real .= $dirFiles[$kFile];
+        return $real.$dirFiles[$kFile];
     }
 
     /**

Tests/Exception/FlattenExceptionTest.php

@@ -294,7 +294,7 @@ class FlattenExceptionTest extends TestCase
 
         // assertEquals() does not like NAN values.
         $this->assertEquals($array[$i][0], 'float');
-        $this->assertNan($array[$i++][1]);
+        $this->assertNan($array[$i][1]);
     }
 
     public function testRecursionInArguments()