A sane variant converting constructor (LEWG 227)

Rationale

See LEWG 227.

Wording

This wording is relative to N4640.

Modify 20.7.3.1 [variant.ctor]/12 as indicated:

template <class T> constexpr variant(T&& t) noexcept(see below );

In the following sections, let is_non_narrowing_convertible_v be an exposition only trait defined as:

template <typename From, typename To>
  constexpr bool is_non_narrowing_convertible_v = see below;

is_non_narrowing_convertible_v<From, To> is true if is_convertible_v<From, To> is true and converting a non-constant object of From to To does not require a narrowing conversion (8.6.4) or a boolean conversion (4.14), otherwise false.

Let T_j be a type that is determined as follows: build an imaginary function FUN(T_i) for each alternative type T_i, where FUN(T_i) shall not participate in overload resolution unless is_non_narrowing_convertible_v<T, T_i> is true. The overload FUN(T_i) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative T_j which is the type of the contained value after construction.

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Modify 20.7.3.3 [variant.assign]/8 as indicated:

template <class T> variant& operator=(T&& t) noexcept(see below );

Let T_j be a type that is determined as follows: build an imaginary function FUN(T_i) for each alternative type T_i, where FUN(T_i) shall not participate in overload resolution unless is_non_narrowing_convertible_v<T, T_i> is true. The overload FUN(T_i) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative T_j which is the type of the contained value after assignment.

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