Proposed resolution:

This wording is relative to N4567.

1. Edit [tuple.cnstr] as indicated:

   template <class... UTypes>
     EXPLICIT constexpr tuple(UTypes&&... u);
   

   -8- Requires: sizeof...(Types) == sizeof...(UTypes).

   […]

   -10- Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) >= 1 and sizeof...(Types) == sizeof...(UTypes) and is_constructible<T_i, U_i&&>::value is true for all i. The constructor is explicit if and only if is_convertible<U_i&&, T_i>::value is false for at least one i.

   […]

   template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);
   

   -15- Requires: sizeof...(Types) == sizeof...(UTypes).

   […]

   -17- Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and is_constructible<T_i, const U_i&>::value is true for all i. The constructor is explicit if and only if is_convertible<const U_i&, T_i>::value is false for at least one i.

   template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);
   

   -18- Requires: sizeof...(Types) == sizeof...(UTypes).

   […]

   -20- Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and is_constructible<T_i, U_i&&>::value is true for all i. The constructor is explicit if and only if is_convertible<U_i&&, T_i>::value is false for at least one i.

   template <class U1, class U2> EXPLICIT constexpr tuple(const pair<U1, U2>& u);
   

   -21- Requires: sizeof...(Types) == 2.

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   -23- Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == 2 and is_constructible<T₀, const U1&>::value is true and is_constructible<T₁, const U2&>::value is true. The constructor is explicit if and only if is_convertible<const U1&, T₀>::value is false or is_convertible<const U2&, T₁>::value is false.

   template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u);
   

   -24- Requires: sizeof...(Types) == 2.

   […]

   -26- Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == 2 and is_constructible<T₀, U1&&>::value is true and is_constructible<T₁, U2&&>::value is true. The constructor is explicit if and only if is_convertible<U1&&, T₀>::value is false or is_convertible<U2&&, T₁>::value is false.