Issue 3531: LWG 3025 broke previous valid code

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

3531. LWG 3025 broke previous valid code

Section: 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview], 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview] Status: New Submitter: Mike Spertus Opened: 2021-03-09 Last modified: 2021-04-20

Priority: 3

View all other issues in [map.overview].

View all issues with New status.

Discussion:

The resolution for LWG 3025⁽ⁱ⁾ enabled code like the following to be accepted:

map m1{{pair{1, 2}, {3, 4}}, less<int>()};

but breaks code that had been previously working like the following

using value_type = pair<const int, int>;

map m2{{value_type{1, 2}, {3, 4}}, less<int>()};

as shown on godbolt.

[Acknowledgment to Tim Song and Arthur O'Dwyer for independently pointing out this case on the LWG mailing list]

[2021-04-20; Reflector poll]

Priority set to 3. Three preferences expressed for Option B, none for A.

Proposed resolution:

This wording is relative to N4878.

We present two partial wording options for std::map, denoted by (A) and (B) below. If the committee accepts one of them, we will complete them to all key-value containers.

(A) Wording option 1: In this option, we restore the deduction guide that was removed in LWG 3025⁽ⁱ⁾ while maintaining the one that was added, demonstrates this working.

Modify 23.4.3.1 [map.overview] as indicated:

[…]
template<class Key, class T, class Compare = less<Key>,
         class Allocator = allocator<pair<const Key, T>>>
  map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
    -> map<Key, T, Compare, Allocator>;
    
template<class Key, class T, class Compare = less<Key>,
         class Allocator = allocator<pair<const Key, T>>>
  map(initializer_list<pair<const Key, T>>, Compare = Compare(), Allocator = Allocator())
    -> map<Key, T, Compare, Allocator>;

template<class InputIterator, class Allocator>
  map(InputIterator, InputIterator, Allocator)
    -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
           less<iter-key-type<InputIterator>>, Allocator>;
           
template<class Key, class T, class Allocator>
  map(initializer_list<pair<Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;           
           
template<class Key, class T, class Allocator>
  map(initializer_list<pair<const Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;
[…]

(B) Wording option 2: This one follows Tim Song's suggestion: "It seems that the cleanest fix is to 1) disallow the initializer_list<value_type> constructors from being used for CTAD, and 2) change the guides to use remove_const_t<Key>." This change has been tested locally with g++ similar to the above godbolt.

Modify 23.4.3.1 [map.overview] as indicated:

[…]
// types
using key_type = Key;
using mapped_type = T;
using value_type = type_identity_t<pair<const Key, T>>;
[…]

template<class Key, class T, class Compare = less<remove_const_t<Key>>,
         class Allocator = allocator<pair<const Key, T>>>
  map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
    -> map<remove_const_t<Key>, T, Compare, Allocator>;
    
template<class InputIterator, class Allocator>
  map(InputIterator, InputIterator, Allocator)
    -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
           less<iter-key-type<InputIterator>>, Allocator>;
           
template<class Key, class T, class Allocator>
  map(initializer_list<pair<Key, T>>, Allocator) 
    -> map<remove_const_t<Key>, T, less<remove_const_t<Key>>, Allocator>;           

[…]