The failure initially observed on gimp-2.10.22 source and fails the same way on gimp from git.
Smaller example extracted from https://gitlab.gnome.org/GNOME/gimp/-/blob/master/configure.ac#L2131 $ cat configure.ac AC_INIT([GIMP], [gimp_version], [https://gitlab.gnome.org/GNOME/gimp/issues/new], [gimp]) AC_PROG_CC dnl Taken from gimp as is: ####################### # Check for Linux Input ####################### AC_ARG_WITH(linux-input, [ --without-linux-input don't build linux input event controller module]) have_linux_input="no (linux input support disabled)" if test "x$with_linux_input" != "xno"; then AC_CHECK_HEADER(linux/input.h, AC_CHECK_DECL(KEY_OK, have_linux_input=yes, have_linux_input="no (needs Linux 2.6)", [#include <linux/input.h>])) fi AC_OUTPUT Reproducing: $ autoconf-2.69 && ./configure --host=x86_64-pc-linux-gnu ... configure: creating ./config.status <ok> $ autoconf-2.70_beta2 && ./configure --host=x86_64-pc-linux-gnu ./configure: line 1430: 5: Bad file descriptor checking whether is declared... ./configure: line 1432: ${+y}: bad substitution <fails> "bad substitution" looks like an autoconf macro expansion problem. Warning: I don't understand m4. gimp's use of macros looks underquoted but I would expect behavior to be roughly the same between 2.69 and 2.69c. Does it sound about right? Thank you! -- Sergei