https://git.ruby-lang.org/ruby.git/commit/?id=c3dd3b9553

From c3dd3b95538a641bbffb02993985ce0cbac1b9d6 Mon Sep 17 00:00:00 2001
From: Yusuke Endoh <mame@r...>
Date: Fri, 4 Oct 2019 02:35:10 +0900
Subject: iseq.c (rb_iseq_compile_with_option): dummy parent_iseq for the
 parser

The parsing of `RubyVM::InstructionSequence.compile` does not support an
outer scope currently.  So it specified NULL as parent_iseq for the
parser.  However, it resulted in the following false-positive warning.

```
RubyVM::InstructionSequence.compile(<<END)
  o = Object.new
  o #=> <compiled>:2: warning: possibly useless use of a variable in void context
END
```

This change specifies a dummy empty parent_iseq instead of NULL, which
suppresses the false positive.

diff --git a/iseq.c b/iseq.c
index d7e8cce..fcf38fc 100644
--- a/iseq.c
+++ b/iseq.c
@@ -994,7 +994,14 @@ rb_iseq_compile_with_option(VALUE src, VALUE file, VALUE realpath, VALUE line, c https://github.com/ruby/ruby/blob/trunk/iseq.c#L994
     }
     {
 	const VALUE parser = rb_parser_new();
-	rb_parser_set_context(parser, parent, FALSE);
+        const rb_iseq_t *outer_scope = parent;
+        if (!outer_scope) {
+            VALUE name = rb_fstring_lit("<compiled>");
+            outer_scope = rb_iseq_new(NULL, name, name, Qnil, 0, ISEQ_TYPE_TOP);
+        }
+        VALUE outer_scope_v = (VALUE)outer_scope;
+        rb_parser_set_context(parser, outer_scope, FALSE);
+        RB_GC_GUARD(outer_scope_v);
 	ast = (*parse)(parser, file, src, ln);
     }
 
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