f() { for i; do echo "|$i|"; done; }
x=a

echo Unquoted b c d
f ${x:+b c d}
echo Unquoted "'b c' d"
f ${x:+'b c' d}
echo Unquoted '"b c" d'
f ${x:+"b c" d}

echo Quoted b c d
f "${x:+b c d}"
echo Quoted "'b c' d"
f "${x:+'b c' d}"
echo Quoted '"b c" d'
f "${x:+"b c" d}"
