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.

1 files changed, 8 insertions, 1 deletions

diff --git a/iseq.c b/iseq.c
index d7e8cce821..fcf38fcea8 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
     }
     {
 	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);
     }