Suggested resolution:

Insert a bullet after 7.7.2 [expr.const.core] bullet 2.18 as follows:

• ...
• an invocation of a destructor (11.4.7 [class.dtor]) or a function call whose postfix-expression names a pseudo-destructor (7.6.1.3 [expr.call]), in either case for an object whose lifetime did not begin within the evaluation of E;
• starting the lifetime of an object O such that the lifetime of the complete object of O did not begin within the evaluation of E;
• ...

(From LWG issue 4578.)

Consider:

  union U { int x = 10; float f[2]; };
  U u;

  consteval {
    std::start_lifetime(u.f); // changes active member
  };

  void foo() {
    U u;
    constexpr int x = (std::start_lifetime(u.f), 10);
  }

There is no prohibition against explicitly starting the lifetime of a runtime object during constant evaluation.