shareable_constant_value: is effective only in comment-only line

parse.y

@@ -7999,6 +7999,13 @@ parser_set_compile_option_flag(struct parser_params *p, const char *name, const
 static void
 parser_set_shareable_constant_value(struct parser_params *p, const char *name, const char *val)
 {
+    for (const char *s = p->lex.pbeg, *e = p->lex.pcur; s < e; ++s) {
+	if (*s == ' ' || *s == '\t') continue;
+	if (*s == '#') break;
+	rb_warning1("`%s' is ignored unless in comment-only line", WARN_S(name));
+	return;
+    }
+
     int b = parser_get_bool(p, name, val);
     if (b >= 0) p->ctxt.shareable_constant_value = b;
 }

test/ruby/test_parse.rb

@@ -1178,26 +1178,27 @@ x = __ENCODING__
     assert_warning(/invalid value/) do
       assert_valid_syntax("# shareable_constant_value: invalid-option", verbose: true)
     end
+    assert_warning(/ignored/) do
+      assert_valid_syntax("nil # shareable_constant_value: true", verbose: true)
+    end
     a, b, c = Class.new.class_eval("#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
       # shareable_constant_value: true
       A = [[1]]
       # shareable_constant_value: false
       B = [[2]]
-      C = # shareable_constant_value: true
-        [[3]]
 
-      [A, B, C]
+      [A, B]
     end;
     assert_send([Ractor, :shareable?, a])
     assert_not_send([Ractor, :shareable?, b])
-    assert_send([Ractor, :shareable?, c])
     assert_equal([1], a[0])
     assert_send([Ractor, :shareable?, a[0]])
     a, b = Class.new.class_eval("#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
       # shareable_constant_value: false
-      class X # shareable_constant_value: true
+      class X
+        # shareable_constant_value: true
         A = [[1]]
       end
       B = []