We discovered this in our project working under gcc4.9.2 (-std=c++14) but it is also present in gcc5.2 - whilst not present in clang.

The code below does not copy b1 to b2 properly:
  - b2[0].c.a is 0
  - b1.c.a

Making b1 compiler time constant (by making makeA() function constexpr) "fixes" the problem:

#include <iostream>
 
struct A 
{
    int a;
};
 
struct B
{
    int b;
    A c;
};
 
/*constexpr*/ //<-- contstexpr here "fixes" the problem
A makeA()   
{
    A a = {2};
    return a;
}
 
const B b1 = {1, makeA()};
 
B b2[] = { b1 };
 
 
int main()
{
    std::cout << b2[0].c.a << std::endl; // 0 (should be 2)
    std::cout << b1.c.a << std::endl;    // 2
}

The link to coliru: http://coliru.stacked-crooked.com/a/4f1ef718fea4e76f

The original example was with std::vector and much more complicated structs. We did an effort to simplify example. Simplifying more also "fixes" the problem.

BR,
Piotr Nycz (piotrwn1(at)gmail.com)