front doesn't compile for arrays of immutable data

[Roman D. Boiko]

import std.range;
struct Element {
  //immutable // uncomment to break compilation
   int _i;
  this(int i) { _i = i; }
}
void main() {
  auto arr = [Element.init];
  arr.front;
}
Declaring _i immutable yields the following compilation error:

/usr/include/x86_64-linux-gnu/dmd/phobos/std/array.d(460): Error: 
a[0LU] isn't mutable
sample.d(11): Error: template instance std.array.front!(Element) 
error instantiating

Is this a bug, or I misunderstand something?