POSIX allows any command to be a builtin, as long as a real external command is ...

POSIX allows any command to be a builtin, as long as a real external command is also available which can be executed via execv and whatnot.

  $ echo $BASH_VERSION
  4.4.20(1)-release
  $ type true
  true is a shell builtin

Also:

  $ type test
  test is a shell builtin
  $ type [
  [ is a shell builtin