Need to call `mrb_str_modify()` in `mrb_str_cat_str()`; fix
If `str` and `str2` are the same string object `str->ptr` may be
rewritten by `mrb_str_modify()`.
MODIFY
src/string.c
src/string.c
@@ -2615,6 +2615,9 @@ mrb_str_cat_cstr(mrb_state *mrb, mrb_value str, const char *ptr)
MRB_API mrb_value
mrb_str_cat_str(mrb_state *mrb, mrb_value str, mrb_value str2)
{
+ if (mrb_str_ptr(str) == mrb_str_ptr(str2)) {
+ mrb_str_modify(mrb, mrb_str_ptr(str));
+ }
return mrb_str_cat(mrb, str, RSTRING_PTR(str2), RSTRING_LEN(str2));
}

Need to call `mrb_str_modify()` in `mrb_str_cat_str()`; fix #4018

If `str` and `str2` are the same string object `str->ptr` may be
rewritten by `mrb_str_modify()`.