Use typed spaces when the line is inside the here documents

Use first method instead of square brackets to support 2.5 and 2.6 versions

lib/irb/ruby-lex.rb

@@ -207,7 +207,11 @@ def set_auto_indent(context)
           last_line = lines[line_index]&.byteslice(0, byte_pointer)
           code += last_line if last_line
           @tokens = self.class.ripper_lex_without_warning(code, context: context)
-          corresponding_token_depth = check_corresponding_token_depth
+          corresponding_token_depth = if heredoc_scope?(line_index)
+                                        lines[line_index][/^ */].length
+                                      else
+                                        check_corresponding_token_depth
+                                      end
           if corresponding_token_depth
             corresponding_token_depth
           else
@@ -817,5 +821,12 @@ def check_termination_in_prev_line(code, context: nil)
     end
     false
   end
+
+  private
+
+  def heredoc_scope?(line_index)
+    tokens = @tokens.first(line_index).select { |t| [:on_heredoc_beg, :on_heredoc_end].include?(t.event) }
+    tokens[-1]&.event == :on_heredoc_beg
+  end
 end
 # :startdoc:

test/irb/test_ruby_lex.rb

@@ -466,9 +466,9 @@ def test_do_corresponding_to_loop
     def test_heredoc_with_indent
       input_with_correct_indents = [
         Row.new(%q(<<~Q), nil, 0, 0),
-        Row.new(%q({), nil, 0, 0),
-        Row.new(%q(  #), nil, 0, 0),
-        Row.new(%q(}), nil, 0, 0),
+        Row.new(%q({), 0, 0, 0),
+        Row.new(%q(  #), 2, 0, 0),
+        Row.new(%q(}), 0, 0, 0)
       ]
 
       lines = []