Is this nogc? dmd and gdc disagree

[rcorre via Digitalmars-d-learn]

I recently tried compiling enumap with GDC, and found that it 
disagrees with DMD on whether a function is @nogc. Here's a 
semi-reduced test-case:

---
import std.range;
import std.traits    : EnumMembers;
import std.typecons  : tuple, staticIota;
import std.algorithm : map;

struct Enumap(K, V)
   if(EnumMembers!K == staticIota!(0, EnumMembers!K.length))
{
   enum length = EnumMembers!K.length;
   private V[length] _store;

   ref auto opIndex(K key) inout {
     return _store[key];
   }

   @nogc auto byKeyValue() const {
     return only(EnumMembers!K).map!(key => tuple(key, this[key]));
   }
}

@nogc void main() {
     import std.typecons  : tuple;
     import std.algorithm : map;

     enum E { a, b, c, d };
     immutable elements = Enumap!(E, int)([1,2,3,4]);

     auto pairs = elements.byKeyValue.map!(pair => tuple(pair[0], 
pair[1] + 1));
}
---

GDC claims that byKeyValue() allocates a closure, but DMD is just 
fine with me calling it @nogc. I'm inclined to agree with GDC 
here, unless DMD is doing some magic so that actually doesn't 
allocate a closure.