When bash is not in POSIX mode, it violates the standard.

  $ ll /bin/sh
  lrwxrwxrwx. 1 root root 4 Nov 24 08:40 /bin/sh -> bash

  $ cat s1
  #!/bin/sh
  alias p=printf
  p hello\\n

  $ cat s2
  #!/bin/bash
  alias p=printf
  p world\\n

  $ ./s1
  hello

  $ ./s2
  ./s2: line 3: p: command not found