ripper/lexer.rb: nested indented heredoc

* ext/ripper/lib/ripper/lexer.rb (on_heredoc_dedent): fix for
  nested indedented here documents, where `Elem`s are nested too.
  [ruby-core:80977] [Bug #13536]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58545 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/ripper/lib/ripper/lexer.rb

@@ -67,7 +67,7 @@ class Ripper
 
     def on_heredoc_dedent(v, w)
       @buf.last.each do |e|
-        if e.event == :on_tstring_content
+        if Elem === e and e.event == :on_tstring_content
           if (n = dedent_string(e.tok, w)) > 0
             e.pos[1] += n
           end

test/ripper/test_lexer.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+begin
+  require_relative 'dummyparser'
+  require 'test/unit'
+  ripper_test = true
+  module TestRipper; end
+rescue LoadError
+end
+
+class TestRipper::Lexer < Test::Unit::TestCase
+  def test_nested_dedent_heredoc
+    bug = '[ruby-core:80977] [Bug #13536]'
+    str = <<~'E'
+    <<~"D"
+    #{
+    <<~"B"
+    this must be a valid ruby
+    B
+    }
+    D
+    E
+    assert_equal(str, Ripper.tokenize(str).join(""), bug)
+  end
+end