front doesn't compile for arrays of immutable data

[Roman D. Boiko]

@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?