This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

2368. Differences in relational and three-way constant comparisons

Section: 7.7  [expr.const]     Status: CD5     Submitter: Richard Smith     Date: 2017-11-11

[Accepted as a DR at the February, 2019 meeting.]

According to 7.7 [expr.const] bullets 2.21 and 2.22, the characteristics of three-way and relational comparisons that disqualify them as constant expressions are different:

These are not equivalent, with odd results:

  struct A {
    int a;
  private:
    int b;
    constexpr auto f() { return &a < &b; }   // not constant 
    constexpr auto g() { return &a <=> &b; } // returns unspecified value 
  };

Similarly,

  struct B { int n; };
  struct C : B { int m; } c;
  constexpr auto x = &c.n < &c.m;   // not constant 
  constexpr auto y = &c.n <=> &c.m; // returns unspecified value

The three-way rule seems to be the correct one, but additional wording is needed in 7.6.9 [expr.rel] to specify the relational ordering within a single object: addresses of subobjects of the same complete object should be weakly ordered, and when restricted to subobjects that are not permitted to have the same address, should be totally ordered.

Notes from the October, 2018 teleconference:

The consensus of CWG was to make the 3-way operator cases non-constant, as the relational cases are.

Proposed resolution (November, 2018):

Change 7.7 [expr.const] bullets 2.22 and 2.23 as follows, merging tbe bullets:

An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (6.9.1 [intro.execution]), would evaluate one of the following expressions: