It's "aggregate initialization" [0]

  struct S {
      int x;
      struct Foo {
          int i;
          int j;
          int a[3];
      } b;
  };
  S s1 = { 1, { 2, 3, {4, 5, 6} } };

0 - https://en.cppreference.com/w/cpp/language/aggregate_initial...