From e2c9544ed5a4c608a59219d6bcda6de5ad555758 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Fri, 6 Mar 2020 11:15:50 -0700
Subject: [PATCH] bc: fix `hash-set` key replacement for `eq?` values

---
 pkgs/racket-test-core/tests/racket/hash.rktl | 12 ++++++++++++
 racket/src/racket/src/hash.c                 |  5 +++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/pkgs/racket-test-core/tests/racket/hash.rktl b/pkgs/racket-test-core/tests/racket/hash.rktl
index f712b7d8de..83e5b9bcdd 100644
--- a/pkgs/racket-test-core/tests/racket/hash.rktl
+++ b/pkgs/racket-test-core/tests/racket/hash.rktl
@@ -565,6 +565,18 @@
 (check-concurrent-gc-of-keys (lambda (ht proc)
                                (equal-hash-code ht)))
 
+;; ----------------------------------------
+;; Make sure a new `equal?`-based key is used when the "new" value is
+;; `eq?` to the old one:
+
+(let ()
+  (define ht (hash))
+  (define f (string-copy "apple"))
+  (define g (string-copy "apple"))
+  (define ht2 (hash-set (hash-set ht f 1) g 1))
+  (test 1 hash-count ht2)
+  (test #t eq? (car (hash-keys ht2)) g))
+
 ;; ----------------------------------------
 
 (report-errs)
diff --git a/racket/src/racket/src/hash.c b/racket/src/racket/src/hash.c
index e2cd468313..337c1f6912 100644
--- a/racket/src/racket/src/hash.c
+++ b/racket/src/racket/src/hash.c
@@ -3467,8 +3467,9 @@ Scheme_Hash_Tree *scheme_hash_tree_set_w_key_wraps(Scheme_Hash_Tree *tree, Schem
              return empty_hash_tree[2];
         } else
           return tree;
-      } else if (SAME_OBJ(val, mzHAMT_VAL(in_tree, pos))) {
-        /* Shortcut: setting to the current value */
+      } else if (SAME_OBJ(val, mzHAMT_VAL(in_tree, pos))
+                 && SAME_OBJ(key, in_tree->els[pos])) {
+        /* Shortcut: setting to the current key and value */
         return tree;
       } else
         return hamt_set(tree, h, 0, key, val, 0);