POSIX.1-2017
12.2 Utility Syntax Guidelines
Guideline 10:
The first --
--argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '--' character.
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02