dmd 2.093.1: duplicate method definitions ignored altogether

[kdevel via Digitalmars-d-learn]

~~~tsnf.d
import std.stdio: writeln;

struct S {
   string s;
   string toString ()
   {
      return __PRETTY_FUNCTION__ ~ `: ` ~ s;
   }
   string toString () const // def #1
   {
      return __PRETTY_FUNCTION__ ~ `: ` ~ s;
   }
   const string toString () // def #2
   {
      return __PRETTY_FUNCTION__ ~ `: ` ~ s;
   }
}
void foo (T) (in ref T t) // fancy way of declaring t const
{
   writeln (t);
}

int main ()
{
   S s;
   writeln (s);
   foo (s);
   return 0;
}
~~~

$ dmd tsnf
$ ./tsnf
string tsnf.S.toString():
const(S)("")

expected output: none. The compiler should have rejected the code 
after the duplicate definition def #2. dmd 2.093.1 ignores both 
definitions instead. Is this a bug or a bug?