enable builtin bugs in bash 5.2

[Emanuele Torre]

Since bash 5.2,  enable mkdir  is equivalent to  enable -f mkdir mkdir.

But, if you use the -n flag, and mkdir is not a loaded builtin, it will
also be equivalent to  enable -f mkdir mkdir  (so the -n flag is
ignored), and the newly loaded foo will be enabled instead of disabled:

  $ ./bash -c 'cd ./examples/loadables; enable -n mkdir; enable -a' |
  > grep mkdir
  enable mkdir

If you call  enable -n mkdir  again, mkdir will be disabled:

  $ ./bash -c 'cd ./examples/loadables; enable -n mkdir; enable -n mkdir
  > enable -a' | grep mkdir
  enable -n mkdir

I think that this is not intended and that, if the -n flag is specified,
enable should only try to disable all the loadable builtins named as the
argument, and skip, with a warning message, arguments that are not
loaded builtins; instead of trying to load (as enabled) arguments, that
are not loaded builtins, from files in the current directory and
BASH_LOADABLES_PATH as it does now.

Another "bug" caused by the new behaviour of  enable name  is that
enable ''  will be equivalent to  enable -f '' ''  that causes bash to
call dlopen() with "" as filename that, at least on GNU/Linux, seems to
have the same behaviour as NULL as filename, so dlopen() will use
current process's executable as file, and  enable ''  will try to load
the  _struct  symbol from the bash binary as a builtin, instead of
erroring:

  ./bash: line 1: enable: cannot find _struct in shared object : ./bash: 
undefined symbol: _struct
  ./bash: line 1: enable: : not a shell builtin

The same thing happens when using  enable -n ''  since -n just is
ignored when the argument ('') is not a loaded builtin.

Also note that  enable -f '' ''  prints two error lines in bash 5.2,
while, in bash 5.1, it only prints one.

  $ bash -c "enable -f '' ''"
  bash: line 1: enable: cannot find _struct in shared object : bash: undefined 
symbol: _struct

o/
 emanuele6