Here is some documentation relevant to the problem:
http://en.cppreference.com/w/cpp/language/aggregate_initialization
In c++11 your code is invalid. In c++14 it is valid again.
In C++11 adding a default initialization prevents braced init from being valid. In C++14, it does not.
A way to solve your problem in C++11 would be to write a constructor with the value for a and the b value with a default.