Building the curses module against the native SVr4 curses — for example on illumos/Solaris with ./configure --with-curses=curses — fails, because Modules/_cursesmodule.c uses several optional functions unconditionally that old SVr4 curses does not provide.

Implicit-declaration errors:

wattr_get, wattr_set, wattr_on, wattr_off, wcolor_set — the window.attr_get(), attr_set(), attr_on(), attr_off() and color_set() methods (the X/Open attr_t API).
slk_attr_on, slk_attr_off, slk_attr_set, slk_color — the soft-label attribute functions.
scr_set — SVr4 has scr_dump()/scr_restore()/scr_init() but not scr_set(), so gating the whole family on scr_dump alone is not enough.

Other optional curses functions (is_pad, resizeterm, …) are already probed and #ifdef-gated; these are not.

Two more problems surface once those are gated:

update_lines_cols() is compiled only under HAVE_CURSES_RESIZETERM/HAVE_CURSES_RESIZE_TERM, but it is used unconditionally (e.g. by set_term()), so a curses without resizeterm() fails to link.
The SVr4 <curses.h> does typedef char bool;, which clashes with C's bool (a macro for _Bool from <stdbool.h>) and fails to compile.

test.test_curses also needs to tolerate such a curses: skip the tests that use the now-optional functions, and treat the native curses of NetBSD and illumos/Solaris as having a broken newterm() (they crash on repeated newterm()/delscreen(), like ncurses before 6.5).

Linked PRs

gh-154055: Gate optional curses functions absent on old SVr4 curses #154057