Change in indirect expansion behavior from 4.4 to 5.0

[Jason Miller]

Gentoo linux, GNU bash, version 5.0.18(1)-release (x86_64-pc-linux-gnu)

On the above vesion of bash, the following script will not run the echo command
and print an error.  On bash 4.4 it appears to  treat the ${!foo} the same as
expanding an unset variable and thus outputs "bar":

  unset foo
  echo ${!foo} bar

"shopt -s compat44" does not seem to make a difference

-Jason