Possible bug in std.algorithm.map

[Robert Jacques]

On Sat, 29 Jan 2011 11:12:28 -0500, Magnus Lie Hetland <mlh at idi.ntnu.no>  
wrote:

> Hi!
>
> Just read Andrei Alexandrescu's new book, and I'm starting to experiment  
> with using D in my algorithms research. Loved the book, and I'm loving  
> the language so far :D
>
> I just hit a snag, though ... I was doing something simple, for which my  
> prototype code (in Python) was
>
>    d, u = max((D(u,v), v) for v in V)
>
> I first started writing it explicitly with loops, but it got a bit too  
> verbose for my taste. Thought I'd use map and reduce, perhaps (although  
> I'm still not sure if that's practical, as I'm reducing with max, but  
> I'd like the argmax as well...).
>
> Anyway -- while using attempting to use map, I suddenly got a segfault.  
> As I hadn't really done any crazy stuff with pointers, or circumvented  
> the bounds checks or the like, I was a bit surprised. I have now boiled  
> things down to the following little program:
>
> import std.algorithm;
> void f() {
>      auto x = 0;
>      double g(int z) { // Alt. 1: return int
>          auto y = x;   // Alt. 2: remove this
>          return 0;
>      }
>      auto seq = [1, 2, 3];
>      auto res = map!(g)(seq);
> }
> void main() {
>      f();
> }
>
> When I compile and run this (dmd 2.051, OS X 10.5.8), I get a  
> segmentation fault.
>
> Oddly enough, if I *either* change the return type to int *or* remove  
> the "y = x" line, things work just fine.
>
> Am I correct in assuming this is a bug?
>

Yes, it's Issue 5073.  
(http://d.puremagic.com/issues/show_bug.cgi?id=5073). I've tested your  
test case using the listed patch + DMD 2.051 and it works. The issue used  
to be a bad compile time error in earlier compiler versions but in DMD  
2.051 it turned into a runtime error. The underlying error in DMD has to  
do with an alias of a delegate having a bad hidden pointer. The reason  
commenting out 'auto y = x;' works is that the incorrect hidden pointer is  
never called, thus never causing a segfault (IIRC). Issue 5073's patch  
works by passing delegates by value instead of by alias. Looking over  
Issue 5064, Don is probably right in it being the root cause in DMD, but  
if you just want map to work correctly, you might want to try the patch  
 from 5073.