Fix pp when passed a empty ruby2_keywords-flagged hash as array element (#2966)

This causes problems because the hash is passed to a block not
accepting keywords.  Because the hash is empty and keyword flagged,
it is removed before calling the block.  This doesn't cause an
ArgumentError because it is a block and not a lambda.  Just like
any other block not passed required arguments, arguments not
passed are set to nil.

Issues like this are a strong reason not to have ruby2_keywords
by default.

Fixes [Bug #16519]

This backports 28d31ead34 and
0ea759eac9, but needed to be modified
for 2.7 as 2.7 will perform empty keyword to positional hash
conversion for required arguments, which will happen if "v" in the
seplist method is empty when yielded.

Co-authored-by: NARUSE, Yui <nurse@users.noreply.github.com>

lib/pp.rb

@@ -223,7 +223,16 @@ class PP < PrettyPrint
         else
           sep.call
         end
-        yield(*v)
+        case v.last
+        when Hash
+          if Hash.ruby2_keywords_hash?(v.last)
+            yield(*v, **{})
+          else
+            yield(*v)
+          end
+        else
+          yield(*v)
+        end
       }
     end
 

test/test_pp.rb

@@ -176,6 +176,11 @@ class PPSingleLineTest < Test::Unit::TestCase
     assert_equal("{1=>1}", PP.singleline_pp({ 1 => 1}, ''.dup)) # [ruby-core:02699]
     assert_equal("[1#{', 1'*99}]", PP.singleline_pp([1]*100, ''.dup))
   end
+
+  def test_hash_in_array
+    assert_equal("[{}]", PP.singleline_pp([->(*a){a.last}.ruby2_keywords.call(**{})], ''.dup))
+    assert_equal("[{}]", PP.singleline_pp([Hash.ruby2_keywords_hash({})], ''.dup))
+  end
 end
 
 class PPDelegateTest < Test::Unit::TestCase