split Error - no overload matches

[forkit]

This code will not compile.

Error: no overload matches for `split`

However, if I uncomment the //import std.uni : isWhite;

then it will compile.

I don't understand. I thought 'import std;' would be sufficient 
here??

//  ----

module test;
@safe:

import std;

void main()
{
     //import std.uni : isWhite; // need to uncomment this for it 
to compile.
     writeln("Learning D is fun".split!isWhite);
}

// -------