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three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>
Section: 25.5.4.4 [move.iter.cons], 25.5.1.4 [reverse.iter.cons] Status: C++23 Submitter: Casey Carter Opened: 2020-04-26 Last modified: 2023-11-22
Priority: 2
View all other issues in [move.iter.cons].
View all issues with C++23 status.
Discussion:
Despite that reverse_iterator<int*>
and reverse_iterator<const int*>
are comparable with
<=>
, three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>
is false
. This unfortunate state of affairs results from the absence of constraints on reverse_iterator
's
converting constructor: both convertible_to<reverse_iterator<int*>, reverse_iterator<const int*>>
and convertible_to<reverse_iterator<const int*>, reverse_iterator<int*>>
evaluate to true
,
despite that reverse_iterator<int*>
's converting constructor template is ill-formed when instantiated
for reverse_iterator<const int*>
. This apparent bi-convertibility results in ambiguity when trying to
determine common_reference_t<const reverse_iterator<int*>&, const reverse_iterator<const int*>&>
,
causing the common_reference
requirement in three_way_comparable_with
to fail.
reverse_iterator
's conversion constructor (and converting
assignment operator, while we're here) correctly so we can use the concept to determine when it's ok to compare
specializations of reverse_iterator
with <=>
.
move_iterator
has similar issues due to its similarly unconstrained conversions. We should fix both similarly.
[2020-07-17; Priority set to 2 in telecon]
[2020-07-26; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.5.4.4 [move.iter.cons] as indicated:
[Drafting note: this incorporates and supercedes the P/R of LWG 3265(i).]
constexpr move_iterator();-1- Effects:
Constructs aValue-initializesmove_iterator
, vingcurrent
.Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of typeIterator
.constexpr explicit move_iterator(Iterator i);-2- Effects:
Constructs aInitializesmove_iterator
, iingcurrent
withstd::move(i)
.template<class U> constexpr move_iterator(const move_iterator<U>& u);-3-
-4- Effects:Mandates:Constraints:U
is convertible toIterator
is_same_v<U, Iterator>
isfalse
andconst U&
modelsconvertible_to<Iterator>
.Constructs aInitializesmove_iterator
, iingcurrent
withu.current
.base()template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);-5-
-6- Effects: AssignsMandates:Constraints:U
is convertible toIterator
is_same_v<U, Iterator>
isfalse
,const U&
modelsconvertible_to<Iterator>
, andassignable_from<Iterator&, const U&>
is modeled.u.current
tobase()current
.
Modify 25.5.1.4 [reverse.iter.cons] as indicated:
template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);-?- Constraints:
-3- Effects: Initializesis_same_v<U, Iterator>
isfalse
andconst U&
modelsconvertible_to<Iterator>
.current
withu.current
.template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);-?- Constraints:
-4- Effects: Assignsis_same_v<U, Iterator>
isfalse
,const U&
modelsconvertible_to<Iterator>
, andassignable_from<Iterator&, const U&>
is modeled.u.current
tobase()current
. -5- Returns:*this
.