import std.stdio;
import std.conv;

struct Foo
{
    string toString() { return to!string(x); }
    alias x this;
    
    int x;
    int y;   // comment this out and the bug is gone
}

void main()
{
    auto foo = Foo();
    writeln(foo);
}

If you comment out "int y;" the bug is gone, otherwise you get back this little thingy:

D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\conv.d(100): Error: template std.conv.toImpl(T,S) if (!implicitlyConverts!(S,T) && isSomeString!(T) && isInputRange!(Unqual!(S)) && isSomeChar!(ElementType!(S))) does not match any function template declaration
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\conv.d(100): Error: template std.conv.toImpl(T,S) if (!implicitlyConverts!(S,T) && isSomeString!(T) && isInputRange!(Unqual!(S)) && isSomeChar!(ElementType!(S))) cannot deduce template function from argument types !(int)(Foo)
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\conv.d(100): Error: template instance errors instantiating template
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\format.d(1683): Error: template instance std.conv.to!(int).to!(Foo) error instantiating
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\format.d(328):        instantiated from here: getNthInt!(Foo)
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\stdio.d(678):        instantiated from here: formattedWrite!(LockingTextWriter,immutable(char),Foo)
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\stdio.d(1511):        instantiated from here: write!(Foo,char)
test.d(18):        instantiated from here: writeln!(Foo)
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\format.d(328): Error: template instance std.format.getNthInt!(Foo) error instantiating
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\stdio.d(678):        instantiated from here: formattedWrite!(LockingTextWriter,immutable(char),Foo)
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\stdio.d(1511):        instantiated from here: write!(Foo,char)
test.d(18):        instantiated from here: writeln!(Foo)
Failed: dmd -d -debug -w -wi -unittest -v -o- "test.d" -I"." >test.d.deps