true;  ``; echo $?
false; ``; echo $?
# UNFIXED BUG. bash sets $? to 127:
true;  `""`; echo $?
# bash sets $? to 127:
false; `""`; echo $?
true;  `     `; echo $?
false; `     `; echo $?

true;  $(); echo $?
false; $(); echo $?
# bash sets $? to 127:
true;  $(""); echo $?
# bash sets $? to 127:
false; $(""); echo $?
true;  $(     ); echo $?
false; $(     ); echo $?

exec ''; echo $?
echo Not reached