Set evaluation result to _ without clobbering the debuggee's _

lib/debug/thread_client.rb

@@ -434,16 +434,24 @@ def frame_eval_core src, b, binding_location: false
       [:return_value,     "_return"],
     ]
 
-    def frame_eval src, re_raise: false, binding_location: false
+    def frame_eval src, re_raise: false, binding_location: false, update_last_value: false
       @success_last_eval = false
 
-      b = current_frame&.eval_binding || TOPLEVEL_BINDING
+      frame = current_frame
+      b = frame&.eval_binding || TOPLEVEL_BINDING
 
-      special_local_variables current_frame do |name, var|
+      special_local_variables frame do |name, var|
         b.local_variable_set(name, var) if /\%/ !~ name
       end
 
+      # eval_binding is a dup that shares the frame's locals, so setting an
+      # existing `_` would overwrite the debuggee's own variable.
+      if frame && !(frame.binding || TOPLEVEL_BINDING).local_variable_defined?(:_)
+        b.local_variable_set(:_, @last_value)
+      end
+
       result = frame_eval_core(src, b, binding_location: binding_location)
+      @last_value = result if update_last_value
 
       @success_last_eval = true
       result
@@ -1045,19 +1053,19 @@ def wait_next_action_
 
           case eval_type
           when :p
-            result = frame_eval(eval_src)
+            result = frame_eval(eval_src, update_last_value: true)
             puts "=> " + color_pp(result, 2 ** 30)
             if alloc_path = ObjectSpace.allocation_sourcefile(result)
               puts "allocated at #{alloc_path}:#{ObjectSpace.allocation_sourceline(result)}"
             end
           when :pp
-            result = frame_eval(eval_src)
+            result = frame_eval(eval_src, update_last_value: true)
             puts color_pp(result, SESSION.width)
             if alloc_path = ObjectSpace.allocation_sourcefile(result)
               puts "allocated at #{alloc_path}:#{ObjectSpace.allocation_sourceline(result)}"
             end
           when :call
-            result = frame_eval(eval_src)
+            result = frame_eval(eval_src, update_last_value: true)
           when :display, :try_display
             failed_results = []
             eval_src.each_with_index{|src, i|

test/console/debugger_local_test.rb

@@ -19,6 +19,53 @@ def test_locals_added_in_locals_are_accessible_between_evaluations
           type "c"
         end
       end
+
+      def test_evaluated_result_is_set_as_underscore_local
+        debug_code(program) do
+          type "_"
+          assert_line_text(/nil/)
+          type "3 * 3"
+          type "foo = _"
+          type "9 == foo"
+          assert_line_text(/true/)
+          type "c"
+        end
+      end
+
+      def test_display_does_not_update_underscore_local
+        debug_code(program) do
+          type "display 42"
+          type "3 * 3"
+          type "display"
+          type "_"
+          assert_line_text(/9/)
+          type "c"
+        end
+      end
+    end
+
+    class UnderscoreDefinedByProgramTest < ConsoleTestCase
+      def program
+        <<~RUBY
+        1| _ = :mine
+        2| binding.b
+        3| p _
+        4| binding.b
+        5| nil
+        RUBY
+      end
+
+      def test_underscore_defined_by_program_is_not_clobbered
+        debug_code(program) do
+          type "c"
+          type "3 * 3"
+          type "_"
+          assert_line_text(/:mine/)
+          type "c"
+          assert_line_text(/:mine/)
+          type "c"
+        end
+      end
     end
 
     class RaisedTest < ConsoleTestCase

test/console/irb_test.rb

@@ -84,6 +84,20 @@ def test_irb_console_config_activates_irb
       ENV["RUBY_DEBUG_IRB_CONSOLE"] = nil
     end
 
+    def test_irb_console_evaluated_result_is_set_as_underscore_local
+      debug_code(program, remote: false) do
+        type 'irb'
+        type "_"
+        assert_line_text(/nil/)
+        type "3 * 3"
+        assert_raw_line_text 'irb:rdbg(main):003> 3 * 3'
+        type "foo = _"
+        type "9 == foo"
+        assert_line_text(/true/)
+        type "c"
+      end
+    end
+
     private
 
     # assert_line_text ignores the prompt line, so we can't use it to assert the prompt transition