Why does BinaryHeap sometime cause compile-error in foeach?

[Shigeki Karita]

https://dpaste.dzfl.pl/cd605899d050

why this code cannot convert to foreach (over Structs and Classes 
with Ranges).

     auto h = new BinaryHeap!(int[])(new int[0]);
     typeof(h).stringof.writeln;
     static assert(isInputRange!(typeof(h)));
     h.insert(3);
     h.insert(1);
     h.insert(2);
     // Error: invalid foreach aggregate `h`
     // foreach (e; h) e.writeln;
     for (; !h.empty; h.popFront()) {
       auto e = h.front();
       e.writeln;
     }


https://dlang.org/spec/statement.html#foreach-with-ranges