Copy constructor for Bar and Frop are silently ignored. When codebase is large with Bar having a significant number of elements, it becomes very confusing why copy constructor of Bar and Frop are getting ignored. Perhaps Foo could as well be imported from a library module.

////
import std.stdio;
struct Frop {
  this (ref inout(Frop) frop) {
    writeln("Frop copy constructor");
  }
}
struct Foo {
  this(this) {
    writeln("Foo postblit constructor");
  }
}
struct Bar {
  Foo foo;
  Frop frop;
  this (ref inout(Bar) bar) {
    writeln("Bar copy constructor");
    foo = bar.foo;
    frop = bar.frop;
  }
}
void main() {
  Bar bar;
  Bar bar2 = bar;
}