https://git.ruby-lang.org/ruby.git/commit/?id=d1ef4fd08e

From d1ef4fd08e60adcbcb4feeb55f767ff3d80b65a0 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Sat, 30 Nov 2019 00:15:29 +0900
Subject: Make single line pattern matching void expression

Instead of returning `nil`, raise a syntax error if its value is
used.  [Feature #16355]

diff --git a/parse.y b/parse.y
index c1df306..15c9b41 100644
--- a/parse.y
+++ b/parse.y
@@ -10740,6 +10740,17 @@ value_expr_check(struct parser_params *p, NODE *node) https://github.com/ruby/ruby/blob/trunk/parse.y#L10740
 	  case NODE_RETRY:
 	    return void_node ? void_node : node;
 
+	  case NODE_CASE3:
+	    if (!node->nd_body || nd_type(node->nd_body) != NODE_IN) {
+		compile_error(p, "unexpected node");
+		return NULL;
+	    }
+	    if (node->nd_body->nd_body) {
+		return NULL;
+	    }
+	    /* single line pattern matching */
+	    return void_node ? void_node : node;
+
 	  case NODE_BLOCK:
 	    while (node->nd_next) {
 		node = node->nd_next;
diff --git a/test/ruby/test_pattern_matching.rb b/test/ruby/test_pattern_matching.rb
index 50bab51..5308ec3 100644
--- a/test/ruby/test_pattern_matching.rb
+++ b/test/ruby/test_pattern_matching.rb
@@ -1261,12 +1261,12 @@ END https://github.com/ruby/ruby/blob/trunk/test/ruby/test_pattern_matching.rb#L1261
   ################################################################
 
   def test_modifier_in
-    assert_nil (1 in a)
+    1 in a
     assert_equal 1, a
     assert_raise(NoMatchingPatternError) do
       {a: 1} in {a: 0}
     end
-    assert_valid_syntax "p(({} in {a:}), a:\n 1)"
+    assert_syntax_error("if {} in {a:}; end", /void value expression/)
     assert_syntax_error(%q{
       1 in a, b
     }, /unexpected/, '[ruby-core:95098]')
-- 
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