void main() @safe {
    import std.stdio: writeln;
    writeln;
}

dmd 2.067alpha gives:

test.d(3,5): Error: safe function 'D main' cannot call system function 'std.stdio.writeln!().writeln'


Note that this compiles:

void main() @safe {
    import std.stdio: write;
    write('\n');
}