Strange nested construct of structures

robot.c

#include <stdio.h>
#include <string.h>

/* gcc -Wall -Werror -std=c11 -pedantic -o robotc robot.c */

struct Robot_st {
    int pos_x;
    int pos_y;
    struct BatteryStatus_st {
        int capacity;
        int load;
    } battery;
};

int main(int argc, char **argv)
{
    struct Robot_st my_robot = {2, 3, {200, 50}};
    struct BatteryStatus_st battery_snapshot;
    
    memcpy(
        &battery_snapshot,
        &my_robot.battery,
        sizeof(struct BatteryStatus_st)
    );
    
    printf("robot position: %d,%d\n", my_robot.pos_x, my_robot.pos_y);
    printf("battery load: %d%%\n", battery_snapshot.load);
    
    return 0;
}

robot.cpp

#include <stdio.h>
#include <string.h>

/* g++ -Wall -Werror -std=c++11 -pedantic -o robotcpp robot.cpp */

struct Robot_st {
    int pos_x;
    int pos_y;
    struct BatteryStatus_st {
        int capacity;
        int load;
    } battery;
};

int main(int argc, char **argv)
{
    struct Robot_st my_robot = {2, 3, {200, 50}};
    struct Robot_st::BatteryStatus_st battery_snapshot;
    
    memcpy(
        &battery_snapshot,
        &my_robot.battery,
        sizeof(struct Robot_st::BatteryStatus_st)
    );
    
    printf("robot position: %d,%d\n", my_robot.pos_x, my_robot.pos_y);
    printf("battery load: %d%%\n", my_robot.battery.load);
    
    return 0;
}