Uniform and Value-initialization

I try to use value-initialization for members with value-initialization for constructors (I don't know if I really use the good terms...)

So... When I define:

struct A
{
  int a_;
};

I'm able to use:

A a{5};
assert(m.a_==5);   

However, if I want to use the member brace initializer AND an initialization-list constructor

struct B
{
  int b_ {1};
};

This doesn't compile (c++14: http://ift.tt/1BNyaGx):

B b{2};

Here is the error:

prog.cpp:19:7: error: no matching function for call to 'B::B(<brace-enclosed initializer list>)'
  B b{2};
       ^
prog.cpp:19:7: note: candidates are:
prog.cpp:10:8: note: constexpr B::B()
 struct B
        ^
prog.cpp:10:8: note:   candidate expects 0 arguments, 1 provided
prog.cpp:10:8: note: constexpr B::B(const B&)
prog.cpp:10:8: note:   no known conversion for argument 1 from 'int' to 'const B&'
prog.cpp:10:8: note: constexpr B::B(B&&)
prog.cpp:10:8: note:   no known conversion for argument 1 from 'int' to 'B&&'

What is the difference, concept-wise? Many thanks!