Explicit defaulted default constructor and aggregates

How to explain the difference, when I compile #if 0 and #if 1 versions of the following code:

#include <cstdlib>

struct A
{ 
    explicit A() = default; // explicitly defaulted or deleted constructors are allowed for aggregates (since C++11)
#if 1
private :
#endif
    int i;
};

int
main()
{
    A a = {};
    return EXIT_SUCCESS;
}

• for #if 0 all is fine, compilation successful.
• for #if 1 compilation failed with error message:

  error: chosen constructor is explicit in copy-initialization

What is the difference for expression A a = {}; depending on whether A is aggreagate or not?