I'm trying to define a boilerplate mixin for class constructor to generate
code such as:
this(T1 a1, T2 a2){this.a1=a1; this.a2=a2;}

The following works, but fails if one field is from a superclass, with an
error such as:
template instance GenThis!(b, a) GenThis!(b, a) is nested in both B and A.

Any suggestion?

----------
template GenThis(fields...){
  import std.typecons;
  import std.typetuple;
  import std.conv;
  private static string fun(){
    string a="this(";
    string b;
    foreach(i;Iota!(fields.length)) {
      alias field=fields[i];
      auto name=Stringof!field;
      alias T=typeof(field);
      if(i>0) a~=", ";
      a~=T.stringof~" "~name;
      b~="this."~name~"="~name~"; ";
    }
    a~=`){`~b~`}`;
    return a;
  }
  enum GenThis=fun();
}

unittest{
  class A{
    int foo;
    string bar;
    mixin(GenThis!(bar, foo));
  }
  auto a=new A("abc",2);
  assert(a.foo==2 && a.bar=="abc");
}