Fix generational GC for weak references

Fixes issue pointed out in https://bugs.ruby-lang.org/issues/21084#note-7.
The following script crashes:

    wmap = ObjectSpace::WeakMap.new

    GC.disable # only manual GCs
    GC.start
    GC.start

    retain = []
    50.times do
      k = Object.new
      wmap[k] = true
      retain << k
    end

    GC.start # wmap promoted, other objects still young

    retain.clear

    GC.start(full_mark: false)

    wmap.keys.each(&:itself) # call method on keys to cause crash

1 files changed, 23 insertions, 0 deletions

diff --git a/test/ruby/test_weakmap.rb b/test/ruby/test_weakmap.rb
index a2904776bc..1050c74b3d 100644
--- a/test/ruby/test_weakmap.rb
+++ b/test/ruby/test_weakmap.rb
@@ -265,4 +265,27 @@ class TestWeakMap < Test::Unit::TestCase
       10_000.times { weakmap[Object.new] = Object.new }
     RUBY
   end
+
+  def test_generational_gc
+    EnvUtil.without_gc do
+      wmap = ObjectSpace::WeakMap.new
+
+      (GC::INTERNAL_CONSTANTS[:RVALUE_OLD_AGE] - 1).times { GC.start }
+
+      retain = []
+      50.times do
+        k = Object.new
+        wmap[k] = true
+        retain << k
+      end
+
+      GC.start # WeakMap promoted, other objects still young
+
+      retain.clear
+
+      GC.start(full_mark: false)
+
+      wmap.keys.each(&:itself) # call method on keys to cause crash
+    end
+  end
 end