This ctor is actually not supposed to be used, so it gives a
pseudo-narrowing warning. Not having this ctor simply gives a compile error
which makes very little sense to the developer who is simply trying to pass
a commonly used size_t et al.
This is not a move to c++17. If the compiler happens to have support
for c++17 namespace scope resolution, they have been kind enough to
backport it to gnu++14. This limits our support for really old c++14
compilers, but that was limited anyway. GCC 6.1 and clang 3.6 tested.