Warning(s):

source_file.cpp:9:25: warning: suggest braces around initialization of subobject [-Wmissing-braces]
        std::array<int, 3> a1({1, 2, 3});
                               ^~~~~~~
                               {      }
source_file.cpp:10:24: warning: suggest braces around initialization of subobject [-Wmissing-braces]
        std::array<int, 3> a2{1, 2, 3};
                              ^~~~~~~
                              {      }
source_file.cpp:11:27: warning: suggest braces around initialization of subobject [-Wmissing-braces]
        std::array<int, 3> a3 = {1, 2, 3};
                                 ^~~~~~~
                                 {      }
source_file.cpp:17:16: warning: suggest braces around initialization of subobject [-Wmissing-braces]
        as.push_back({1, 2, 3});
                      ^~~~~~~
                      {      }
4 warnings generated.