Why does g++ states, that there is "no matching function for call cbegin(o)" for some valarray<double> o?

Question

Please consider the following code:

using custom_t = std::valarray<unsigned>;
custom_t o;
unsigned acc = std::accumulate(std::cbegin(o), std::cend(o), 0);

g++-5 says

No matching function for call to cbegin(custom_t&)

If I use std::begin(o) and std::end(o) instead, everything works. Is this a compiler bug? The code compiles with Visual Studio 2015.

Answer 1

This is a libstdc++ bug, I've just created https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67374