How to ensure string compatibility In D？

[FrankLike]

On Wednesday, 23 January 2019 at 10:44:51 UTC, Jonathan M Davis 
wrote:
> On Tuesday, January 22, 2019 2:49:00 PM MST bauss via 
> Digitalmars-d-learn wrote:

> toUTFz is the generic solution. toStringz exists specifically

Error: template std.utf.toUTFz cannot deduce function from
  argument types !()(string), candidates are:
E:\D\DMD2\WINDOWS\BIN\..\..\src\phobos\std\utf.d(3070):        
std.utf.toUTFz(P)

I have solved the problem in this way：

import core.sys.windows.windows;
import std.stdio;
import std.string;
import std.conv;

void main()
{
	auto	strA_Z ="CD"w;
	auto type = GetDriveType(tos(to!wstring(strA_Z[0])~":\\"));
	writeln(to!wstring(strA_Z[0])~" is ",type);
}

private auto tos(T)(T str)
{
  version (ANSI)
  {
	writeln("ANSI");
	return cast(const(char)*)(str);
  }
  else
  {
	writeln("Unicode");
	return cast(const(wchar)*)(str);
	
  }
}

Thanks.