From b7c3816fdbb01ab3b42fa23db8352d5cee81cf78 Mon Sep 17 00:00:00 2001
From: "H. S. Teoh" <hsteoh@quickfur.ath.cx>
Date: Wed, 5 Nov 2014 22:03:15 -0800
Subject: [PATCH] Do reflexivity check in assert mode if predicate is claimed
 to be equivalence relation.

---
 std/algorithm.d | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/std/algorithm.d b/std/algorithm.d
index 0c162973e..d52dae1c1 100644
--- a/std/algorithm.d
+++ b/std/algorithm.d
@@ -4663,7 +4663,14 @@ private struct GroupByImpl(alias pred, IsEquivRelation isEquivRelation, Range)
     {
         r = _r;
         if (!empty)
+        {
+            // Check reflexivity if predicate is claimed to be an equivalence
+            // relation.
+            assert(!isEquivRelation || pred(r.front, r.front),
+                   "predicate " ~ pred.stringof ~ " is claimed to be "~
+                   "equivalence relation yet isn't reflexive");
             savePrev();
+        }
     }
     @property bool empty() { return r.empty; }