Struct copy and destruction

[Morlan]

The following code:

//***************************************************
import std.conv, std.stdio;

struct Slice {
	int[] buff;
	this(size_t len) {
		buff = new int[len];
	}
	this(this) {
		buff = buff.dup;
		writeln("postblit");
	}
}

struct S {
	string name;
	Slice slc;
	this(string name) {
		writeln(name, " constructor");
		this.name = name;
	}
	~this() {
		writeln(name, " destructor");
	}
}

void main() {
	auto s1 = S("s1");
	auto s2 = S("s2");
	s1 = s2;
	writeln("after assignment");
}
//***********************************************

produces the following output:

s1 constructor
s2 constructor
postblit
s1 destructor
after assignment
s2 destructor
s2 destructor

Note the "s1 destructor" line after "postblit". I cannot find any justification for the destructor
call in this place either in the TDPL book or in the Language Reference. Is this behaviour to be
expected? If so I would be grateful for the proper reference. Otherwise is it a bug?