std.container.rbtree: Fix support for less predicates taking ref const (dlang#10792)

Author: vpanteleev-sym

std/container/rbtree.d

@@ -747,7 +747,7 @@ private struct RBRange(N)
  * inserted after all existing duplicate elements.
  */
 final class RedBlackTree(T, alias less = "a < b", bool allowDuplicates = false)
-if (is(typeof(binaryFun!less(T.init, T.init))))
+if (is(typeof((ref const T a) => binaryFun!less(a, a))))
 {
     import std.meta : allSatisfy;
     import std.range : Take;
@@ -2260,3 +2260,14 @@ if ( is(typeof(binaryFun!less((ElementType!Stuff).init, (ElementType!Stuff).init
     t.insert([1, 3, 5, 4, 2]);
     assert(t[].equal([5, 4, 3, 2, 1]));
 }
+
+// should support `less` predicate taking `ref const`
+@safe pure unittest
+{
+    struct S
+    {
+        int* value;
+    }
+
+    cast(void) new RedBlackTree!(S, (ref const S a, ref const S b) => a.value > b.value);
+}