Aggregate-paren-init breaks direct-initializing a tuple or optional from {aggregate-member-value}
Ville Voutilainen

Created on 2020-05-01.00:00:00 last changed 2 weeks ago


Date: 2020-05-09.19:39:43

Proposed resolution:

This wording is relative to N4861.

  1. Modify [tuple.cnstr] as indicated:

    template<class... UTypes> constexpr explicit(see below) tuple(UTypes&&... u);

    -11- Constraints: sizeof...(Types) equals sizeof...(UTypes) and sizeof...(Types) ≥ 1 and is_constructible_v<Ti, Ui> is true for all i and conjunction_v<is_aggregate<remove_reference_t<Ti>>, negation<is_same<remove_reference_t<Ti>, remove_reference_t<Ui>>>> is false for all i.

  2. Modify [optional.ctor] as indicated:

    template<class U = T> constexpr explicit(see below) optional(U&& v);

    -22- Constraints: is_constructible_v<T, U> is true, is_same_v<remove_cvref_t<U>, in_place_t> is false, and is_same_v<remove_cvref_t<U>, optional> is false, and conjunction_v<is_aggregate<T>, negation<is_same<T, remove_reference_t<U>>>> is false.

Date: 2020-05-15.00:00:00

[ 2020-05-09; Reflector prioritization ]

Set priority to 2 after reflector discussions.

Date: 2020-05-01.00:00:00

For reference, see this gcc bug report.

Constructing a tuple or optional from an element value of an aggregate is broken in C++20. tuple<c> t({val}); and optional<c> t({val}); invoked a non-forwarding constructor before, but now the perfect-forwarding converting constructors are a match, because the element is constructible from {val}. But it's not convertible, so overload resolution chooses the explicit constructor, and the initialization fails.

Tim Song explains the overload resolution in this reflector discussion.

Now that we understand that C++17 called the non-forwarding conversion constructor, and C++20 tries to use the forwarding conversion constructor, we have the solution. SFINAE away the forwarding conversion constructor when it would convert an aggregate.

This also means that tuple<c> t(0); won't work, which is unfortunate because tuple<c>/optional<c> no longer mirrors what c can do. That's okay; in this LWG issue, we first restore feature parity with C++17, and later, as an extension, enable such initializations so that tuple/optional mirrors what c can do in C++20.

The proposed wording below has been implemented and tested.

Date User Action Args
2020-05-09 19:39:43adminsetmessages: + msg11279
2020-05-03 11:58:58adminsetmessages: + msg11262
2020-05-01 00:00:00admincreate