Initializer list constructor for struct with default values

struct S {
    int a = 1;
    int b = 2;
};

The following both compile with clang 3.5 and do not compile with gcc 4.9.2

S s = {10};
S s2 = {10, 20};

The error from gcc lists the only candidate constructors as the default constructor, the copy constructor, and the move constructor. This implies that adding a default value to a class means it can't be brace initialized (without user-defined constructors), but since clang compiles this without errors or unexpected behavior (s.b == 2), who is right?