In 2.047, a struct could provide a toString that returned a wstring (as opposed to string) and it would get used when passing the struct to writeln. 

As of 2.048 a toString returning wstring is not detected and the struct's typename is returned instead. toString returning string works fine.

Repro:

import std.stdio;
import std.conv;

struct A
{
  wstring toString() { return "blah"; }
}

void main()
{
  A a;
  writeln(a);
  writeln(to!string(a));
  writeln(to!wstring(a));
}

DMD 2.047:
blah
blah
blah

DMD 2.048:
A
blah
blah