Title
<=> for containers should require three_way_comparable<T> instead of <=>
Status
new
Section
[container.requirements.general]
Submitter
Jonathan Wakely

Created on 2020-04-17.00:00:00 last changed 1 month ago

Messages

Date: 2020-04-25.11:24:40

Proposed resolution:

This wording is relative to N4861.

  1. Modify [container.requirements.general], Table [tab:container.opt], as indicated:

    Table 75: Optional container operations [tab:container.opt]
    Expression Return type Operational
    semantics
    Assertion/note
    pre-/post-condition
    Complexity
    a <=> b synth-three-way-result<value_type> lexicographical_compare_three_way(
    a.begin(), a.end(), b.begin(), b.end(),
    synth-three-way)
    Preconditions: Either <=> is defined for
    values of type (possibly const)

    T satisfies three_way_comparable,
    or < is defined for values of type
    (possibly const) T and
    < is a total ordering relationship.
    linear
Date: 2020-04-25.00:00:00

[ 2020-04-25 Issue Prioritization ]

Priority to 2 after reflector discussion.

Date: 2020-04-20.15:27:23

The precondition for <=> on containers is:

"Either <=> is defined for values of type (possibly const) T, or < is defined for values of type (possibly const) T and < is a total ordering relationship."

I don't think <=> is sufficient, because synth-three-way won't use <=> unless three_way_comparable<T> is satisfied, which requires weakly-equality-comparable-with<T, T> as well as <=>.

So to use <=> I think the type also requires ==, or more precisely, it must satisfy three_way_comparable.

The problem becomes clearer with the following example:

#include <compare>
#include <vector>

struct X
{
  friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; }
};

std::vector<X> v(1);
std::strong_ordering c = v <=> v;

This doesn't compile, because despite X meeting the preconditions for <=> in [tab:container.opt], synth-three-way will return std::weak_ordering.

Here is another example:

#include <compare>
#include <vector>

struct X
{
  friend bool operator<(X, X) { return true; } // The return value is intentional, see below
  friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; }
};

std::vector<X> v(1);
std::weak_ordering c = v <=> v;

This meets the precondition because it defines <=>, but the result of <=> on vector<X> will be nonsense, because synth-three-way will use operator< not operator<=> and that defines a broken ordering.

So we're stating a precondition which implies "if you do this, you don't get garbage results" and then we give garbage results anyway.

The proposed resolution is one way to fix that, by tightening the precondition so that it matches what synth-three-way actually does.

History
Date User Action Args
2020-04-25 11:24:40adminsetmessages: + msg11233
2020-04-18 22:12:28adminsetmessages: + msg11228
2020-04-17 00:00:00admincreate