Rationale (October, 2012):

Because this is an expression, not a declaration, the As live until the end of the full-expression.

Issue 1030 clarified that elements of an initializer-list are evaluated in the order they are written, but does that also apply to implied expressions? That is, given:

    struct A { A(); ~A(); };
    struct B { B(int, const A& = A()); ~B(); };
    struct C { B b1, b2; };
    int main() { C{1,2}; }

Do we know that the first B is constructed before the second A? I suppose that's what we want, even though it complicates exception region nesting since the As need to live longer than the B subobject cleanups.