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?

@property ref T front(T)(T[] a)
if (!isNarrowString!(T[]) && !is(T[] == void[]))
{
    assert(a.length, "Attempting to fetch the front of an empty array of " ~
           typeof(a[0]).stringof);
    return a[0];
}

Why is front returned by ref even when it is not possible to do so?

On 06/25/2012 04:27 PM, Roman D. Boiko wrote:
> @property ref T front(T)(T[] a)
> if (!isNarrowString!(T[]) && !is(T[] == void[]))
> {
> assert(a.length, "Attempting to fetch the front of an empty array of " ~
> typeof(a[0]).stringof);
> return a[0];
> }
>
> Why is front returned by ref even when it is not possible to do so?

It should always be possible to do so. What you are experiencing seems to be a compiler bug.

On Monday, June 25, 2012 16:27:20 Roman D. Boiko wrote:
> @property ref T front(T)(T[] a)
> if (!isNarrowString!(T[]) && !is(T[] == void[]))
> {
>      assert(a.length, "Attempting to fetch the front of an empty
> array of " ~
>             typeof(a[0]).stringof);
>      return a[0];
> }
> 
> Why is front returned by ref even when it is not possible to do so?

What do you mean that it's not possible? What's not possible about it?

- Jonathan M Davis

On Monday, 25 June 2012 at 15:39:07 UTC, Jonathan M Davis wrote:
> On Monday, June 25, 2012 16:27:20 Roman D. Boiko wrote:
>> @property ref T front(T)(T[] a)
>> if (!isNarrowString!(T[]) && !is(T[] == void[]))
>> {
>>      assert(a.length, "Attempting to fetch the front of an empty
>> array of " ~
>>             typeof(a[0]).stringof);
>>      return a[0];
>> }
>> 
>> Why is front returned by ref even when it is not possible to do
>> so?
>
> What do you mean that it's not possible? What's not possible about it?
>
> - Jonathan M Davis

Looks like I came to wrong conclusions, so my statement from second post should be ignored. The first post shows the problem.

On Monday, 25 June 2012 at 15:38:09 UTC, Timon Gehr wrote:
> It should always be possible to do so. What you are experiencing seems to be a compiler bug.

I couldn't find it reported before. The closest are:
d.puremagic.com/issues/show_bug.cgi?id=6480
http://d.puremagic.com/issues/show_bug.cgi?id=7258

I guess I should report it?

On Monday, 25 June 2012 at 15:58:14 UTC, Roman D. Boiko wrote:
> On Monday, 25 June 2012 at 15:38:09 UTC, Timon Gehr wrote:
>> It should always be possible to do so. What you are experiencing seems to be a compiler bug.
>
> I couldn't find it reported before. The closest are:
> http://d.puremagic.com/issues/show_bug.cgi?id=6480
> http://d.puremagic.com/issues/show_bug.cgi?id=7258
>
> I guess I should report it?
http://d.puremagic.com/issues/show_bug.cgi?id=8297

On Monday, 25 June 2012 at 14:23:25 UTC, Roman D. Boiko wrote:
> 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?

This is a bug of the compiler, and it is already filed.
http://d.puremagic.com/issues/show_bug.cgi?id=6336

I've already posted a pull request to fix the bug, but it has been hold a while.

Kenji Hara