Return brace-enclosed initializer list as struct

Giving below simplified code compiled with g++ -c test.cpp or g++ -std=c++17 -c test.cpp

#include <cstddef>

struct sd_bus_vtable {
        union {
                struct {
                        size_t element_size;
                } start;
                struct {
                        const char *member;
                        const char *signature;
                } signal;
        } x;
};

sd_bus_vtable get()
{
    return {
        .x = {
            .signal = {
                .member = "",
                .signature= "",
            }
        }
    };
}

It compiles fine on GCC 9.2.0 and clang 5/6, but fails on 8.3.0 or 7.4.0 with below error message:

test.cpp:25:5: error: could not convert ‘{{{"", ""}}}’ from ‘<brace-enclosed initializer list>’ to ‘sd_bus_vtable’
     };

To work around it, the function get() could be changed as below, but it looks not so clean...

sd_bus_vtable get()
{
    struct sd_bus_vtable t = {
        .x = {
            .signal = {
                .member = "",
                .signature= "",
            }
        }
    };
    return t;
}

The question is, is the above code valid or not? If yes, does it trigger some bug in GCC that is fixed in GCC9?