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3347. std::pair<T, U> now requires T and U to be less-than-comparable

Section: 22.3.3 [pairs.spec], 23.3.3.1 [array.overview] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-03 Last modified: 2021-02-25

Priority: 1

View all other issues in [pairs.spec].

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Discussion:

P1614R2 added operator<=> as a hidden friend to std::pair:

friend constexpr common_comparison_category_t<synth-three-way-result<T1>, 
                                              synth-three-way-result<T2>>
  operator<=>(const pair& x, const pair& y) { see below }

That is not a function template, so is not a SFINAE context. If one or both of synth-three-way-result<T1> or synth-three-way-result<T2> is an invalid type then the declaration of operator<=> is ill-formed, and so the specialization std::pair<T1, T2> is ill-formed.

A similar problem exists for std::array.

There are at least two ways to fix this:

1. Constrain the function and delay the use of synth-three-way-result until we know it's valid.

2. Replace the hidden friend with a namespace-scope function template, so invalid synth-three-way-result types cause substitution failure.

The first option is somewhat hard to specify, because current policy is to avoid the use of requires-clauses in most of the library clauses. Even with a requires-clause, the potentially-invalid synth-three-way-result types cannot be used in the function declarator. Furthermore, the operator<=> for std::array is currently specified in Table [tab:container.opt] and so there's nowhere to add a Constraints: element.

The second option would partially revert the P1614R2 changes for std::pair and std::array and bring them closer to what was in C++17. The main motivation for making operator== a hidden friend was to allow it to be defaulted, so that std::pair and std::array would be usable as non-type template parameters. Following the acceptance of P1907 in Belfast it isn't necessary to default it, so we can go back to what was in C++17.

[2019-12-12 Issue Prioritization]

Priority to 1 after reflector discussion.

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4842.

1. Modify 22.2.1 [utility.syn] as indicated:

   [Drafting note: This restores the pre-P1614R2 operator== and uses operator<=> as replacement for operator<, operator<=, operator>, operator>=.]

   […]
   
   // 22.3 [pairs], class template pair
   template<class T1, class T2>
   struct pair;
   
   // 22.3.3 [pairs.spec], pair specialized algorithms
   template<class T1, class T2>
     constexpr bool operator==(const pair<T1, T2>&, const pair<T1, T2>&);
   template<class T1, class T2>
     constexpr common_comparison_category_t<synth-three-way-result<T1>, 
                                            synth-three-way-result<T2>>
     operator<=>(const pair<T1, T2>&, const pair<T1, T2>&);
     
   template<class T1, class T2>
   constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
   […]
   

2. Modify 22.3.2 [pairs.pair] as indicated:

     […]
     constexpr void swap(pair& p) noexcept(see below);
     
     // 22.3.3 [pairs.spec], pair specialized algorithms
     friend constexpr bool operator==(const pair&, const pair&) = default;
     friend constexpr bool operator==(const pair& x, const pair& y)
       requires (is_reference_v<T1> || is_reference_v<T2>)
       { return x.first == y.first && x.second == y.second; }
     friend constexpr common_comparison_category_t<synth-three-way-result<T1>,
                                                   synth-three-way-result<T2>>
       operator<=>(const pair& x, const pair& y) { see below }
   };
   
   template<class T1, class T2>
   pair(T1, T2) -> pair<T1, T2>;
   […]
   

3. Modify 22.3.3 [pairs.spec] as indicated:

   20.4.3 Specialized algorithms [pairs.spec]

   template<class T1, class T2>
     constexpr bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);
   

   -?- Returns: x.first == y.first && x.second == y.second.

   template<class T1, class T2>
   friend constexpr
     common_comparison_category_t<synth-three-way-result<T1>, synth-three-way-result<T2>>
       operator<=>(const pair<T1, T2>& x, const pair<T1, T2>& y);
   

   -1- Effects: Equivalent to:

   if (auto c = synth-three-way(x.first, y.first); c != 0) return c;
   return synth-three-way(x.second, y.second);
   

4. Modify 23.3.2 [array.syn] as indicated:

   [Drafting note: This restores the pre-P1614R2 operator== and uses operator<=> as replacement for operator<, operator<=, operator>, operator>=.]

   namespace std {
   
   // 23.3.3 [array], class template array
   template<class T, size_t N> struct array;
   
   template<class T, size_t N>
     constexpr bool operator==(const array<T, N>& x, const array<T, N>& y);
   template<class T, size_t N>
     constexpr synth-three-way-result<T>
       operator<=>(const array<T, N>& x, const array<T, N>& y);
   
   template<class T, size_t N>
   constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));
   […]
   

5. Modify 23.3.3.1 [array.overview] as indicated:

   [Drafting note: there is no need to add definitions of operator== and operator<=> to [array.spec] because they are defined by Table 71: Container requirements [tab:container.req] and Table 73: Optional container operations [tab:container.opt] respectively.]

     […]
     constexpr T * data() noexcept;
     constexpr const T * data() const noexcept;
     
     friend constexpr bool operator==(const array&, const array&) = default;
     friend constexpr synth-three-way-result<value_type>
       operator<=>(const array&, const array&);
   };
   
   template<class T, class... U>
   array(T, U...) -> array<T, 1 + sizeof...(U)>;
   […]