From 0b93dcca87212556860dbf27c1955d7cbfad8c21 Mon Sep 17 00:00:00 2001
From: Vladimir Panteleev <git@cy.md>
Date: Sat, 19 Apr 2025 09:11:16 +0000
Subject: [PATCH 1/2] std.container.rbtree: Return range elements by ref

---
 std/container/rbtree.d | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/std/container/rbtree.d b/std/container/rbtree.d
index 53697029c97..05a05203d45 100644
--- a/std/container/rbtree.d
+++ b/std/container/rbtree.d
@@ -679,7 +679,7 @@ private struct RBRange(N)
     /**
      * Returns the first element in the range
      */
-    @property Elem front()
+    ref @property Elem front()
     {
         return _begin.value;
     }
@@ -687,7 +687,7 @@ private struct RBRange(N)
     /**
      * Returns the last element in the range
      */
-    @property Elem back()
+    ref @property Elem back()
     {
         return _end.prev.value;
     }
@@ -1036,7 +1036,7 @@ if (is(typeof(binaryFun!less(T.init, T.init))))
      *
      * Complexity: $(BIGOH 1)
      */
-    inout(Elem) front() inout
+    ref inout(Elem) front() inout
     {
         return _begin.value;
     }
@@ -1046,7 +1046,7 @@ if (is(typeof(binaryFun!less(T.init, T.init))))
      *
      * Complexity: $(BIGOH log(n))
      */
-    inout(Elem) back() inout
+    ref inout(Elem) back() inout
     {
         return _end.prev.value;
     }

From ab89679e31e4fef1a6e3a2fba226acf30105029b Mon Sep 17 00:00:00 2001
From: Vladimir Panteleev <git@cy.md>
Date: Sun, 20 Apr 2025 09:58:56 +0000
Subject: [PATCH 2/2] std.container.rbtree: Add warning about modifying
 elements by ref

---
 std/container/rbtree.d | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/std/container/rbtree.d b/std/container/rbtree.d
index 05a05203d45..6ff2a0e9cf8 100644
--- a/std/container/rbtree.d
+++ b/std/container/rbtree.d
@@ -737,6 +737,10 @@ private struct RBRange(N)
  * elements `a` and `b`, $(D less(a, b) == !less(b, a)). $(D less(a, a)) should
  * always equal `false`.
  *
+ * Care should also be taken to not modify elements in the tree (e.g. via `front` /
+ * `back`, which return by `ref`) in a way which would affect the order defined by
+ * the `less` predicate.
+ *
  * If `allowDuplicates` is set to `true`, then inserting the same element more than
  * once continues to add more elements.  If it is `false`, duplicate elements are
  * ignored on insertion.  If duplicates are allowed, then new elements are