There are two issues here:

(1) If libreadline is already linked against ncurses, there is no way that
    the readline module should be linked against ncursesw. This was the
    direct cause of the FreeBSD segfault, so this configuration is also
    prohibited in 2.6/3.1.

(2) Interaction between the readline and curses modules: 2.6/3.1 still
    allow readline+ncurses with curses+ncursesw. To be on the safe side,
    2.7/3.2 enforce all ncurses or all ncursesw.


(1) implies that if your readline module is linked against ncurses in 3.1,
you should find that libreadline is also linked against ncurses.