Consider I have this type:

---

struct Type {
  int a;
  const int& b;
  constexpr Type() : a(1), b(a) {}
};

constexpr Type t1;
constexpr int c11 = t1.a;
constexpr int c12 = t1.b;

---

c11 and c12 are fine for both gcc and clang

Now lets introduce constexpr function which is subject to RVO:

constexpr auto get() {
  return Type();
}

Now try this:

constexpr Type t2 = get();
constexpr int c2 = t2.a;

This code compiles fine with clang but not with gcc.

Live example on godbolt: https://godbolt.org/z/zf3YvebbG

According to [expr.const] and [class.copy.elision] result of constexpr function is necessary RVO and is contant expression, so t2 construction is fine and t2.a is constant expression as well.