Make String#-@ not freeze receiver if called on unfrozen subclass instance

rb_fstring behavior in this case is to freeze the receiver.  I'm
not sure if that should be changed, so this takes the conservative
approach of duping the receiver in String#-@ before passing
to rb_fstring.

Fixes [Bug #15926]

1 files changed, 3 insertions, 0 deletions

diff --git a/string.c b/string.c
index 6bb8a24313..2bf9586e98 100644
--- a/string.c
+++ b/string.c
@@ -2658,6 +2658,9 @@ str_uplus(VALUE str)
 static VALUE
 str_uminus(VALUE str)
 {
+    if (!BARE_STRING_P(str) && !rb_obj_frozen_p(str)) {
+        str = rb_str_dup(str);
+    }
     return rb_fstring(str);
 }