Why is Visual Studio 2013 having trouble with this class member decltype?

Question

#include <vector>

struct C
{
    std::vector<int> v;
    decltype(v.begin()) begin() { return v.begin(); }
    decltype(v.end()) end() { return v.end(); }
};

Clang++ has no problem, but MSVC 2013 gives the following error:

error C2228: left of '.begin' must have class/struct/union

Answer 1

This is a known bug in VS2013, fixed^† in VS2015. The compiler will accept the code if you use a trailing return type instead.

struct C
{
    std::vector<int> v;
    auto begin() -> decltype(v.begin()) { return v.begin(); }
    auto end()   -> decltype(v.end())   { return v.end(); }
};

As the bug report says, another work around is by using:

struct C
{
    std::vector<int> v;
    decltype(std::declval<decltype(v)>().begin()) begin() { return v.begin(); }
    decltype(std::declval<decltype(v)>().end())   end()   { return v.end(); }
};

But as @BenVoigt points out in the comments, read this answer for why the trailing return type should be the preferred option.

---

^† Search the linked page for C++ decltype of class-member access incompletely implemented