The implementation of std::forward

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The problem with the first is that you can write std::forward(x), which doesn’t do what you want, since it always produces lvalue references.

The argument in the second case is a non-deduced context, preventing automatic deduction of the template argument. This forces you to write std::forward<T>(x), which is the right thing to do.

Also, the argument type for the second overload should be typename identity<T>::type& because the input to idiomatic use of std::forward is always an lvalue.

Edit: The standard actually mandates a signature equivalent to this one (which, incidentally, is exactly what libc++ has):

template <class T> T&& forward(typename remove_reference<T>::type& t) noexcept;
template <class T> T&& forward(typename remove_reference<T>::type&& t) noexcept;

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