Why is compilation failing with this selective import?

[Nick Hamann]

I'm not sure if this is a bug or if I'm doing something wrong.
Compilation succeeds and the program runs successfully with this 
code:

import std.stdio;
import std.conv : to;
void main() {
    auto x = std.conv.to!double("7.3");
    writeln(x - 2.2);
}


However, when I change the first line to "import std.stdio : 
writeln;", I instead get:

: dmd main.d
main.d(5): Error: undefined identifier std


I'm running DMD 2.064 on 64-bit Arch Linux.