C++14: Initializing constexpr variables from parameter values

Question

Say I have a class that that can return a constant expression through a constexpr function:

template
struct Foo {
  constexpr int Bar()         


        

Answer 1

§5.20:

The reference does not have a preceding initialization from the point of view of i, though: It's a parameter. It's initialized once ByReference is called.

Let's remove the constexpr from i's declaration and consider an invocation of ByReference in its entirety:

template<int N>
constexpr int ByReference(const Foo<N> &f) {
    int i = f.Bar();
    return i;
}

constexpr int j = ByReference(Foo<0>());

This is fine, since f does have preceding initialization. The initializer of f is a constant expression as well, since the implicitly declared default constructor is constexpr in this case (§12.1/5).
Hence i is initialized by a constant expression and the invocation is a constant expression itself.