Error(s):

source_file.cpp: In function ‘int main()’:
source_file.cpp:4:37: sorry, unimplemented: non-trivial designated initializers not supported
     int a[6] = { [4] = 29, [2] = 15 };
                                     ^
source_file.cpp:4:37: sorry, unimplemented: non-trivial designated initializers not supported
source_file.cpp:4:9: warning: unused variable ‘a’ [-Wunused-variable]
     int a[6] = { [4] = 29, [2] = 15 };
         ^