merge revision(s) 3592b24cdc07ed89eecb39161f21fe721a89a5de: [Backport #19531]

	ObjectSpace::WeakMap: clean inverse reference when an entry is
	 re-assigned

	[Bug #19531]

	```ruby
	wmap[1] = "A"
	wmap[1] = "B"
	```

	In the example above, we need to remove the `"A" => 1` inverse reference
	so that when `"A"` is GCed the `1` key isn't deleted.
	---
	 test/ruby/test_weakmap.rb | 17 +++++++++
	 weakmap.c                 | 91 ++++++++++++++++++++++++++++++++++++++---------
	 2 files changed, 91 insertions(+), 17 deletions(-)

1 files changed, 17 insertions, 0 deletions

diff --git a/test/ruby/test_weakmap.rb b/test/ruby/test_weakmap.rb
index 6455034743..7fc956dfae 100644
--- a/test/ruby/test_weakmap.rb
+++ b/test/ruby/test_weakmap.rb
@@ -196,4 +196,21 @@ class TestWeakMap < Test::Unit::TestCase
       GC.compact
     end;
   end
+
+  def test_replaced_values_bug_19531
+    a = "A".dup
+    b = "B".dup
+
+    @wm[1] = a
+    @wm[1] = a
+    @wm[1] = a
+
+    @wm[1] = b
+    assert_equal b, @wm[1]
+
+    a = nil
+    GC.start
+
+    assert_equal b, @wm[1]
+  end
 end