import std.stdio;

class Inner {}

class Outer {
  Inner inner; alias inner this;
  this(Inner i) { inner = i; }
}

void main() {
  Inner[] inners = [];
  inners ~= new Inner;
  inners ~= new Outer(new Inner); // Appends null
  
  foreach(inner; inners){
    writeln(">> ", inner);
  }
}


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Output from https://run.dlang.io/ with all dmd compilers:

Up to      2.083.1: Success with output:
-----
>> onlineapp.Inner
>> onlineapp.Inner
-----

Since      2.084.0: Success with output:
-----
>> onlineapp.Inner
>> null
-----