[Issue 7381] New: Make auto tail-const

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7381

           Summary: Make auto tail-const
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: jmdavisProg at gmx.com


--- Comment #0 from Jonathan M Davis <jmdavisProg at gmx.com> 2012-01-27 14:40:46 PST ---
IFTI was recently changed with arrays to be tail-const. It would be useful if
we could do the same with auto. To quote dsimcha from the newsgroup (
http://www.digitalmars.com/d/archives/digitalmars/D/auto_Top-level_Const_Immutable_152877.html
):

The changes made to IFTI in DMD 2.057 are great, but they reveal another 
hassle with getting generic code to play nice with const.

import std.range, std.array;

ElementType!R sum(R)(R range) {
     if(range.empty) return 0;
     auto ans = range.front;
     range.popFront();

     foreach(elem; range) ans += elem;
     return ans;
}

void main() {
     const double[] nums = [1, 2, 3];
     sum(nums);
}

test.d(8): Error: variable test9.sum!(const(double)[]).sum.ans cannot 
modify const
test.d(14): Error: template instance test9.sum!(const(double)[]) error 
instantiating

Of course this is fixable with an Unqual, but it requires the programmer 
to remember this every time and breaks for structs with indirection. 
Should we make `auto` also strip top-level const from primitives and 
arrays and, if const(Object)ref gets in, from objects?

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