Why is creating of if Expressions not allowed?

[sighoya via Digitalmars-d-learn]

Why

auto GenIf()()
{
    return mixin("if(true) { return true;} else {return false;}");
}
public bool testFunction2()
{
    GenIf!();
}


gives me:

onlineapp.d-mixin-3(3): Error: expression expected, not if
onlineapp.d(8): Error: template instance `onlineapp.GenIf!()` 
error instantiating