tab completing dir contents fails if dir name includes '[]'

[John Van Sickle]

bash version: 5.0.2(1)-release
OS: Debian Sid, Linux 4.20.10, x86_64

Tab completion to list the contents of a directory will fail if the
directory includes '[]' in its name.

Example:

$ mkdir foo 'foo[]'; touch foo/baz 'foo[]'/baz

$ ls foo/<tab here>
   Expands to 'foo/baz'

But

$ ls foo\[\]/<tab here>
   Expands to 'foo[]//' and does not list 'baz' or the directory contents.


I had a knowledgeable person in #bash (freenode) confirm this bug and they
said it happens in the development branch as well.
It does work correctly in bash 4.4.12.


Thanks,
John Van Sickle