I plan to change it to a more liberal license for the first stable release (I should add a note to the readme). I still have to choose which one. The GPLv3 was chosen as an interim conservative option.

One way to help me choose one of the most liberal licenses is to get your company to join the sponsorship program ;)

Anyways, I and many others use plenty of GPLv3 software every day, let's agree to disagree on the "useless" part...

Similar library with MIT, if that is your cup of tea: https://github.com/rsms/immutable-cpp

For most people? I highly doubt that. But it might make it unuseable for you.

The vast majority of C++ programmers in the world aren't writing open source software.

You are free to use GPL libraries in proprietary code as long as you don't distribute it.

Any sources? I'm not sure it is that obvious.

According to https://adtmag.com/Blogs/WatersWorks/2014/01/Worldwide-Devel..., in 2014 there were 18 million coders world wide (so few!) of which 11 million were professional and 7.5 million hobbyists. Hobbyists does not have to mean Open Source, but professional does not have to mean proprietary either. At the same time, half of all open source software was written by professionals (http://www.techrepublic.com/article/for-50-percent-of-develo...). I am not aware that there are any complete stats on how many open source developers exist. And one should not forget that also a professional C++ programmer can use a library as this for a FOSS project.

We are getting OT. It is okay to ask with respects to the licence, but it is not okay to act like a GPL project is worthless.