Preserve an existing _ local when creating a WorkSpace

lib/irb/workspace.rb

@@ -87,7 +87,7 @@ def initialize(*main)
         end
       end
 
-      @binding.local_variable_set(:_, nil)
+      @binding.local_variable_set(:_, nil) unless @binding.local_variable_defined?(:_)
     end
 
     # The Binding of this workspace

test/irb/test_debugger_integration.rb

@@ -426,6 +426,22 @@ def test_irb_command_can_check_local_variables
       assert_include(output, '"FooBar"')
     end
 
+    def test_irb_console_mode_keeps_program_defined_underscore
+      @envs.merge!("RUBY_DEBUG_IRB_CONSOLE" => "true")
+      write_ruby <<~'ruby'
+        _ = :mine
+        binding.irb
+      ruby
+
+      output = run_ruby_file do
+        type "debug"
+        type "_"
+        type "continue"
+      end
+      assert_match(/^:mine\r?$/, output)
+      assert_no_match(/^nil\r?$/, output)
+    end
+
     def test_help_command_is_delegated_to_the_debugger
       write_ruby <<~'ruby'
         binding.irb

test/irb/test_workspace.rb

@@ -38,6 +38,18 @@ def test_code_around_binding
       IRB.conf.delete(:USE_COLORIZE)
     end
 
+    def test_workspace_keeps_existing_underscore
+      _ = :mine
+      IRB::WorkSpace.new(binding)
+      assert_equal(:mine, _)
+    end
+
+    def test_workspace_defines_underscore_as_nil_when_absent
+      workspace = IRB::WorkSpace.new(binding)
+      assert_equal(true, workspace.binding.local_variable_defined?(:_))
+      assert_nil(workspace.binding.local_variable_get(:_))
+    end
+
     def test_code_around_binding_with_existing_unreadable_file
       pend 'chmod cannot make file unreadable on windows' if windows?
       pend 'skipped in root privilege' if Process.uid == 0