Where does the __1 symbol come from when using LLVM's libc++?

Question

I see a fair amount of questions like Apple Mach-O Linker (Id) Error and Undefined symbols in cryptopp at IOS 64-bit project. The problem is usually described as:

Answer 1

It is from C++11 inlined namespaces

libc++ has something like

namespace std {
    inline namespace __1 {
        ....

more at What are inline namespaces for?