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### 3595. Exposition-only classes `proxy` and `postfix-proxy` for `common_iterator` should be fully `constexpr`

Section: 23.5.4.4 [common.iter.access], 23.5.4.5 [common.iter.nav] Status: WP Submitter: Hewill Kang Opened: 2021-09-18 Last modified: 2021-10-14

Priority: Not Prioritized

View all other issues in [common.iter.access].

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Discussion:

LWG 3574 added `constexpr` to all member functions and friends of `common_iterator` to make it fully `constexpr`, but accidentally omitted the exposition-only classes `proxy` and `postfix-proxy` defined for some member functions. We should make these two classes fully `constexpr`, too.

[2021-09-24; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 23.5.4.4 [common.iter.access] as indicated:

   >     decltype(auto) operator->() const
   >       requires see below;

   > > -3- […]

   > > -4- […]

   > > -5- Effects:

   > > 1. (5.1) — […]

   > > 2. (5.2) — […]

   > > 3. (5.3) — Otherwise, equivalent to: `return proxy(*get<I>(v_));` where `proxy` is the exposition-only class:

   > >    >     class proxy {
   > >    >       iter_value_t<I> keep_;
   > >    >       constexpr proxy(iter_reference_t<I>&& x)
   > >    >         : keep_(std::move(x)) {}
   > >    >     public:
   > >    >       constexpr const iter_value_t<I>* operator->() const noexcept {
   > >    >         return addressof(keep_);
   > >    >       }
   > >    >     };

2. Modify 23.5.4.5 [common.iter.nav] as indicated:

   >     decltype(auto) operator++(int);

   > > -4- […]

   > > -5- Effects: If I models forward_iterator, equivalent to:

   > > […]

   > > Otherwise, if […]

   > > […]

   > > Otherwise, equivalent to:

   > > >     postfix-proxy p(**this);
   > > >     ++*this;
   > > >     return p;

   > > where `postfix-proxy` is the exposition-only class:

   > > >     class postfix-proxy {
   > > >       iter_value_t<I> keep_;
   > > >       constexpr postfix-proxy(iter_reference_t<I>&& x)
   > > >         : keep_(std::forward<iter_reference_t<I>>(x)) {}
   > > >     public:
   > > >       constexpr const iter_value_t<I>& operator*() const noexcept {
   > > >         return keep_;
   > > >       }
   > > >     };