I have seen -- used in the compgen command. For example: compgen -W "foo bar baz" -- b What is the meaning of the --.

More precisely, a double dash (--) is used in bash built-in commands and many other commands to signify the end of command options, after which only positional parameters are accepted.
Example use: lets say you want to grep a file for the string "-v" - normally "-v" will be considered the option to reverse the matching meaning (only show lines that do not match), but with -- you can grep for "-v" like this:

grep -- -v file