Is it specified in the C++11 standard that std::begin(Container&&) returns const_iterator?

As you can see in http://en.cppreference.com/w/cpp/iterator/begin the interesting overloads are:

template<class C> auto begin(C& c) -> decltype(c.begin());
template<class C> auto begin(const C& c) -> decltype(c.begin());

and std::vector<int>&& can only bind to the second overload (so returns const_iterator).

Let's try to analyze what happens, step by step:

1. You're calling std::begin(std::vector<int>&&), but std::begin has no overload that takes an rvalue:

   template< class C > 
   auto begin( C& c ) -> decltype(c.begin());
   
   template< class C > 
   auto begin( const C& c ) -> decltype(c.begin());
   

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2. Due to reference collapsing, a temporary (xvalue) will only bind to a const lvalue reference:

   If you call Fwd with an xvalue, we again get Type&& as the type of v. This will not allow you to call a function that takes a non-const lvalue, as an xvalue cannot bind to a non-const lvalue reference. It can bind to a const lvalue reference, so if Call used a const&, we could call Fwd with an xvalue.

   (From the linked answer).

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3. Therefore, the

    template<class C> auto begin(const C& c) -> decltype(c.begin());
   

   overload is being called, which returns a const iterator.

   Why?

   Because std::begin(v) calls v.begin(), which returns a const_iterator when called on const instances of std::vector.