dn.d

import std.stdio;

// secret sauce: https://dlang.org/spec/class.html#alias-this

struct DayNum { 
	int dn; 
	alias dn this; // this line makes DayNum look like an int with dn as the value
}

DayNum next(DayNum d) {
	int inty = d + 1; // this is ok, a DayNum can be used as an int
	return DayNum(inty); // conversion from int to DayNum still needs to be explicit
}

void main() {
	auto n = next( DayNum(2) );
	//auto n = next( 2 ); // this will not compile, 2 is not a DayNum
	writeln(n);
}

Same answer basically:

struct bondNotLive
{
	Date settle ;
	Date maturity ;
	double coupon ;
	double accrued ;
}

struct bond
{
	immutable bondNotLive bnl;
	double cleanPrice;
	double dirtyPrice;
	double yieldAnnual;

	alias bnl this; // <---
}

void main() {
	auto bn = bondNotLive(Date(2020, 4, 5), Date(2020,4,7), 12, 13);
	auto b = bond(bn, 2, 3, 4);
	writeln(b.coupon); // ok, says 12
}