Currently, an opApply function cannot be @nogc:

struct NumberRange {
  int begin;
  int end;

  int opApply(int delegate(ref int) @nogc operations) @nogc const 
{
    int result = 0;

    for (int number = begin; number != end; ++number) {
      result = operations(number);

      if (result) {
        break;
      }
    }
    return result;
  }
}

void main() {
  import std.stdio;
  foreach (element; NumberRange(3, 7)) { // line 21
    write(element, ' ');
  }
}

When compiled with 2.066.0:

opapply.d(21): Error: function opapply.NumberRange.opApply (int 
delegate(ref int) @nogc operations) const is not callable using 
argument types (int delegate(ref int __applyArg0) @system)

It would be nice if this code could be allowed, preferably by inference, otherwise by allowing something like

foreach (element; NumberRange(3, 7) @nogc) { ... }

Discussion here: http://forum.dlang.org/thread/fqaskxirvcxrdevmacdo@forum.dlang.org