Add a regression test for opt_plus with unknown type (#5878)

Merged-By: maximecb <maximecb@ruby-lang.org>

test/ruby/test_yjit.rb

@@ -1,4 +1,8 @@
 # frozen_string_literal: true
+#
+# This set of tests can be run with:
+# make test-all TESTS='test/ruby/test_yjit.rb' RUN_OPTS="--yjit-call-threshold=1"
+
 require 'test/unit'
 require 'envutil'
 require 'tmpdir'
@@ -453,6 +457,27 @@ class TestYJIT < Test::Unit::TestCase
     RUBY
   end
 
+  def test_getivar_opt_plus
+    assert_no_exits(<<~RUBY)
+      class TheClass
+        def initialize
+            @levar = 1
+        end
+
+        def get_sum
+            sum = 0
+            # The type of levar is unknown,
+            # but this still should not exit
+            sum += @levar
+            sum
+        end
+      end
+
+      obj = TheClass.new
+      obj.get_sum
+    RUBY
+  end
+
   def test_super_iseq
     assert_compiles(<<~'RUBY', insns: %i[invokesuper opt_plus opt_mult], result: 15)
       class A