Index: hash.c
===================================================================
--- hash.c	(リビジョン 67857)
+++ hash.c	(作業コピー)
@@ -607,10 +607,14 @@
 {
     st_table *new_tab;
     ar_table_entry *entry;
-    const unsigned size = RHASH_AR_TABLE_SIZE(hash);
+    unsigned size;
     st_index_t i;
 
-    if (!RHASH_AR_TABLE_P(hash) || size < RHASH_AR_TABLE_MAX_SIZE) {
+    if (!RHASH_AR_TABLE_P(hash)) return;
+
+    size = RHASH_AR_TABLE_SIZE(hash);
+
+    if (size < RHASH_AR_TABLE_MAX_SIZE) {
         return;
     }
 
@@ -824,6 +828,11 @@
     st_data_t value = 0, old_key;
     st_hash_t hash_value = do_hash(key);
 
+    if (UNLIKELY(!RHASH_AR_TABLE_P(hash))) {
+        /* `#hash` changes ar_table -> st_table */
+        return -1;
+    }
+
     if (RHASH_AR_TABLE_SIZE(hash) > 0) {
         bin = find_entry(hash, hash_value, key);
         existing = (bin != RHASH_AR_TABLE_MAX_BOUND) ? TRUE : FALSE;
@@ -872,6 +881,11 @@
     unsigned bin = RHASH_AR_TABLE_BOUND(hash);
     st_hash_t hash_value = do_hash(key);
 
+    if (UNLIKELY(!RHASH_AR_TABLE_P(hash))) {
+        /* `#hash` changes ar_table -> st_table */
+        return -1;
+    }
+
     hash_ar_table(hash); /* prepare ltbl */
 
     bin = find_entry(hash, hash_value, key);
@@ -900,7 +914,12 @@
 ar_lookup(VALUE hash, st_data_t key, st_data_t *value)
 {
     st_hash_t hash_value = do_hash(key);
-    unsigned bin = find_entry(hash, hash_value, key);
+    unsigned bin;
+    if (UNLIKELY(!RHASH_AR_TABLE_P(hash))) {
+        /* `#hash` changes ar_table -> st_table */
+        return st_lookup(RHASH_ST_TABLE(hash), key, value);
+    }
+    bin = find_entry(hash, hash_value, key);
 
     if (bin == RHASH_AR_TABLE_MAX_BOUND) {
         return 0;
@@ -920,6 +939,10 @@
     unsigned bin;
     st_hash_t hash_value = do_hash(*key);
 
+    if (UNLIKELY(!RHASH_AR_TABLE_P(hash))) {
+        /* `#hash` changes ar_table -> st_table */
+        return st_delete(RHASH_ST_TABLE(hash), key, value);
+    }
 
     bin = find_entry(hash, hash_value, *key);
 
Index: test/ruby/test_hash.rb
===================================================================
--- test/ruby/test_hash.rb	(リビジョン 67857)
+++ test/ruby/test_hash.rb	(作業コピー)
@@ -1733,4 +1733,62 @@
       super
     end
   end
+
+  def test_ar2st
+    # insert
+    obj = Object.new
+    obj.instance_variable_set(:@h, h = {})
+    def obj.hash
+      10.times{|i| @h[i] = i}
+      0
+    end
+    def obj.inspect
+      'test'
+    end
+    h[obj] = true
+    assert_equal '{0=>0, 1=>1, 2=>2, 3=>3, 4=>4, 5=>5, 6=>6, 7=>7, 8=>8, 9=>9, test=>true}', h.inspect
+
+    # delete
+    obj = Object.new
+    obj.instance_variable_set(:@h, h = {})
+    def obj.hash
+      10.times{|i| @h[i] = i}
+      0
+    end
+    def obj.inspect
+      'test'
+    end
+    def obj.eql? other
+      other.class == Object
+    end
+    obj2 = Object.new
+    def obj2.hash
+      0
+    end
+
+    h[obj2] = true
+    h.delete obj
+    assert_equal '{0=>0, 1=>1, 2=>2, 3=>3, 4=>4, 5=>5, 6=>6, 7=>7, 8=>8, 9=>9}', h.inspect
+
+    # lookup
+    obj = Object.new
+    obj.instance_variable_set(:@h, h = {})
+    def obj.hash
+      10.times{|i| @h[i] = i}
+      0
+    end
+    def obj.inspect
+      'test'
+    end
+    def obj.eql? other
+      other.class == Object
+    end
+    obj2 = Object.new
+    def obj2.hash
+      0
+    end
+
+    h[obj2] = true
+    assert_equal true, h[obj]
+  end
 end