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:

• ...

• a three-way comparison (7.6.8 [expr.spaceship]), comparing pointers that do not point to the same complete object or to any subobject thereof;

• a relational (7.6.9 [expr.rel]), or equality (7.6.10 [expr.eq]) operator where the result is unspecified;

• ...

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.

[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:

• a three-way comparison (7.6.8 [expr.spaceship]) comparing pointers that do not point to the same complete object or to any subobject thereof;

• a relational (7.6.9 [expr.rel]) or equality (7.6.10 [expr.eq]) operator where the result is unspecified;

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.