Rationale (July, 2019)

The example is valid; the constant evaluation is the entire initialization constexpr A a1, thus the temporary bound to a1.temporary started its lifetime within the constant evaluation.

There is implementation divergence with respect to an example like:

  constexpr int f(int &r) { r *= 9; return r - 12; }
  struct A { int &&temporary; int x; int y; };

  constexpr A a1 = { 6, f(a1.temporary), a1.temporary }; // #1 

Some implementations accept this code and others say that a1.temporary is not a constant expression in the initializer at #1.