Comparison problem with case modification expansion in bash-4.3

[Corentin Peuvrel]

Hi,

I think I found a bug in bash-4.3 (4.3.42 in fedora 21), that seems to exists 
for a long time (at least from 4.1.2 in Centos 6).

I found a fairly simple way to reproduce it : you have to initialize a variable 
to an empty string, and do a comparaison to it while doing the case 
modification expansion ${var,,}

This doesn't work :
# foo="" ; [[ bar == *"${foo,,}"* ]] && echo ok

While this works fine :
# foo="" ; [[ bar == *"${foo}"* ]] && echo ok
ok

If you unset the variable, it works as we want :
# unset foo ; [[ bar == *"${foo,,}"* ]] && echo ok
ok

If the variable is not empty, it also works normally :
# foo="A" ; [[ bar == *"${foo,,}"* ]] && echo ok
ok

I hope it's not already known, but I couldn't find anything on the net.

Thank you,

Corentin Peuvrel