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

Shell Parameter Expansion documentation for bash is here. No mention of +:, though it does mention :+:

${parameter:+word}
If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.