Why can't std::tuple be element-wise constructed with a std::tuple of compatible types?

Question

I can\'t initialize std::tuple elements element-wise from a std::tuple of compatible types. Why doesn\'t it work as with boost::tuple?

Answer 1

Overloads (4) and (5) are poorer matches than (3) when passed a tuple& : they are const& and && overloads, while (3) matches exactly through the magic of perfect forwarding.

(3) is valid because your Foo(U&&) constructor is overly greedy.

Add SFINAE checks to Foo(U&&) so that it fails to match when it fails to build:

template <class U,
  std::enable_if_t<std::is_convertible<U,int>{},int>* =nullptr
>
Foo(U &&u) : val(std::forward<U>(u)) {}

The rvalue case should, however, work or be ambiguous. Looking at the error log of your live example, the only error I see is with the lvalue one.