seen in a focal test rebuild, all architctures:
https://launchpad.net/ubuntu/+archive/test-rebuild-20200810-focal/+build/19795016
g++ -std=c++11 -DHAVE_CONFIG_H -I.. -I../gdk -I../gtk -pthread -pthread -I/usr/include/atkmm-1.6 -I/usr/include/giomm-2.4 -I/usr/lib/x86_64-linux-gnu/giomm-2.4/include -I/usr/include/pangomm-1.4 -I/usr/lib/x86_64-linux-gnu/pangomm-1.4/include -I/usr/include/glibmm-2.4 -I/usr/lib/x86_64-linux-gnu/glibmm-2.4/include -I/usr/include/cairomm-1.0 -I/usr/lib/x86_64-linux-gnu/cairomm-1.0/include -I/usr/include/sigc++-2.0 -I/usr/lib/x86_64-linux-gnu/sigc++-2.0/include -I/usr/include/gtk-3.0/unix-print -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0 -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/fribidi -I/usr/include/harfbuzz -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -DDEMOCODEDIR=\"""\" -Wdate-time -D_FORTIFY_SOURCE=2 -Wall -DGLIBMM_DISABLE_DEPRECATED -DGIOMM_DISABLE_DEPRECATED -DGTKMM_DISABLE_DEPRECATED -DGDKMM_DISABLE_DEPRECATED -g -O2 -fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -c -o gtk-demo/gtkmm_demo-example_appwindow.o `test -f 'gtk-demo/example_appwindow.cc' || echo './'`gtk-demo/example_appwindow.cc
gtk-demo/demowindow.cc:54:1: error: ‘demo_columns’ function uses ‘auto’ type specifier without trailing return type
54 | const auto& demo_columns()
| ^~~~~
gtk-demo/demowindow.cc:54:1: note: deduced return type only available with ‘-std=c++14’ or ‘-std=gnu++14’
make[4]: *** [Makefile:733: gtk-demo/gtkmm_demo-demowindow.o] Error 1
make[4]: *** Waiting for unfinished jobs....
