I tried to keep it C++11, but generalized lambdas are critical in important use cases (see nop::Variant). Besides, GCC and Clang have solid C++14 support. C++17 is another story... ;-)
The problem is not compiler support but the fact library is going to get used with other code base that is still C++11. Typically projects build system will enforce this and it’s hard to change without full evaluation of all dependencies. Is it possible to add support for C++11 and turn of these additional features that might not get used?