betterC question

[Dibyendu Majumdar via Digitalmars-d-learn]

I have simple test program:

import core.stdc.stdio : printf;
void test() {
    int* a;
    printf("a == null %d\n", a == null);
}

int function() fp = test;

extern (C) void main() {
    fp();
}

Why do I get:

\d\dmd-2.092.1\windows\bin64\dmd.exe -betterC tests.d
tests.d(5): Error: printf cannot be interpreted at compile time, 
because it has no available source code

This is on Windows