std.utf.decode("dlang", 1)

[Algo via Digitalmars-d-learn]

void main()
{
       import std.utf;
       decode("dlang", 1);
}

Error: template std.utf.decode cannot deduce function from
argument types !()(string, int), candidates are:
D:\msc\D\dmd2\windows\bin\..\..\src\phobos\std\utf.d(924):
std.utf.decode(S)(auto ref S str, ref size_t index) if
(!isSomeString!S && isRandomAccessRange!S && hasSlicing!S &&
hasLength!S && isSomeChar!(ElementType!S))
D:\msc\D\dmd2\windows\bin\..\..\src\phobos\std\utf.d(942):
std.utf.decode(S)(auto ref S str, ref size_t index) if
(isSomeString!S)

why doesn't "decode(S)(auto ref S str, ref size_t index) if
(isSomeString!S)" correspond?