Why are designated initializers not implemented in g++

As I noted in a comment, G++ doesn't support C99 standard designated initialisers, but it does support the GNU extension to C90 which allows designated initialisers. So this doesn't work:

union value_t {
    char * v_cp;
    float v_f;
};
union value_t my_val = { .v_f = 3.5f };

But this does:

union value_t my_val = { v_f: 3.5f };

This seems to be a bad interaction of co-ordination between the C and C++ standards committees (there is no particularly good reason why C++ doesn't support the C99 syntax, they just haven't considered it) and GCC politics (C++ shouldn't support C99 syntax just because it's in C99, but it should support GNU extension syntax that achieves exactly the same thing because that's a GNU extension that can be applied to either language).

I ran into this same problem today. g++ with -std=c++11 and c++14 does support designated initializers, but you can still get a compilation error "test.cxx:78:9: sorry, unimplemented: non-trivial designated initializers not supported" if you don't initialize the struct in the order in which it's members have been defined. As an example

struct x
{
    int a;
    int b;
};

// This is correct
struct x x_1 = {.a = 1, .b = 2};
// This will fail to compile with error non-trivial designated initializer
struct x x_2 = {.b = 1, .a = 2};