What does “plus colon” (“+:”) mean in shell script expressions?

Question

What does this mean?

if ${ac_cv_lib_lept_pixCreate+:} false; then :
  $as_echo_n \"(cached) \" >&6
else
  ac_check_lib_save_LIBS=$LIBS

Answer 1

Simple examples will prove

I check for presence of a parameter TEST, if present echo "Yes" else I echo "No"

openvas:~$ ${TEST+:} false  &&  echo "yes" || echo "no"
no
openvas:~$ TEST=1
openvas:~$ ${TEST+:} false  &&  echo "yes" || echo "no"
yes
openvas:~$ 

If you see, the parameter TEST is evaluated and it is actually unset, so it returns false and exit the path and goes to the OR Once I set the same, and test again, it flows to the echo part (continued &&) since it returns true

Refer: this and that