The signals2 Git repo does not have issues enabled, so I thought I'd ping the list about this. Trying to compile any program that includes boost/signals2 with -fno-exceptions fails as of 1.64: #include <boost/signals2.hpp> // boost/signals2/slot.hpp will also reproduce the error int main() { } g++ prog.cc -I<path to Boost 1.64.0 headers> -std=c++11 -fno-exceptions View it on Wandbox: https://wandbox.org/permlink/vZVpF5bb8E9K7sDz I was also able to reproduce this locally on the develop branch. It appears this is because the slot_base lock function is not defined when exceptions are disabled, but this is not checked when it lock is called in `slot_template.hpp`. I am working on a fix, but I would like to know what the path moving forward for the fix would be. Signals2 is currently unusable in 1.64 with exceptions turned off.