Proposed resolution:

This wording is relative to N5046.

1. Modify [obj.lifetime] as indicated:

   template<class T>
     constexpr void start_lifetime(T& r) noexcept;
   

   -1- Mandates: `T` is a complete type and an implicit-lifetime ([basic.types]) aggregate ([dcl.init.aggr]).

   -?- Constant When: `r` refers to a non-volatile object whose lifetime began within the evaluation of the core constant expression ([expr.const.core]) which includes the invocation of `start_lifetime`, or a sub-object of such an object.

   -2- Effects: If the object referenced by `r` is already within its lifetime ([basic.life]), no effects. Otherwise, begins the lifetime of the object referenced by `r`.

As currently specified, the following code is well-formed:

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

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

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

The `start_lifetime(r)` when evaluated at compile time, should require that `r` is a subobject of the object created during the enclosing constant evaluation.