Extended Description

Libc++ implements the two standard constructors 1) 'tuple(tuple<Up...> const&)' and 2) 'tuple(tuple<Up...>&&)' as a single generic overload 'tuple(Tuple&&)'. This can cause a slight differences in overload resolution.

For example the call to 'tuple(tuple<...>{})' will first consider ctor #​2. However if this constructor SFINAE's aware then ctor #​1 should be considered.
Since libc++ only provides a single overload it will only consider #​2. This breaks the following code:

std::tuple<int&>(std::tuple<int&&>(42));