Avoid an MSVC crash on dry fountains etc (xFleury)

map_cell has a bit field "dungeon_feature_type _feat:8", which MSVC
makes signed.  However, some dungeon_feature_type enumerators are
greater than 127 (beginning with DNGN_DRY_FOUNTAIN = 128).  That results
in some bounds errors when encountering one of those features.

According to the C++ standard, 8 bits should be large enough to hold the
enum (§7.2 [dcl.enum] paragraph 7), so the values in the bit field
should compare equal to the corresponding enumerators (§9.6 [class.bit]
paragraph 4). So it sounds like MSVC is in the wrong. Still, work around
it by casting to uint8_t and then to the full enum type.

At this point maybe we should just use a uint8_t instead of a bit field
(since we have to cast anyway).  However, that doesn't look as nice in gdb...

1 files changed, 4 insertions, 1 deletions

diff --git a/crawl-ref/source/map_knowledge.h b/crawl-ref/source/map_knowledge.h
index c044538c69..d8ad79e3ab 100644
--- a/crawl-ref/source/map_knowledge.h
+++ b/crawl-ref/source/map_knowledge.h
@@ -120,7 +120,10 @@ struct map_cell
 
     dungeon_feature_type feat() const
     {
-        return _feat;
+        // Ugh; MSVC makes the bit field signed even though that means it can't
+        // actually hold all the enum values. That seems to be in contradiction
+        // of the standard (§9.6 [class.bit] paragraph 4) but what can you do?
+        return dungeon_feature_type(uint8_t(_feat));
     }
 
     unsigned feat_colour() const