Is this code valid or not? GCC and Clang disagree

The following code gives me different results on GCC and Clang. Who's right?

union Foo {
    struct {
        int a;
        int b;
    };
};

struct Bar {
    Bar(void) : test{.a = 1, .b = 2} { }
    Foo test;
};

I get the following error with GCC (and it compiles fine with Clang):

arcanis@/tmp # g++ -std=c++11 x.cpp 
x.cpp: In constructor ‘Bar::Bar()’:
x.cpp:9:36: error: too many initializers for ‘Foo’
     Bar(void) : test{.a = 1, .b = 2} { }
                                    ^