str_replace: self should not be shared and nofree

src/string.c

@@ -1368,14 +1368,17 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
   long len;
 
   len = RSTR_LEN(s2);
-  if (RSTR_SHARED_P(s2)) {
-  L_SHARE:
   if (RSTR_SHARED_P(s1)) {
     str_decref(mrb, s1->as.heap.aux.shared);
   }
   else if (!RSTR_EMBED_P(s1) && !RSTR_NOFREE_P(s1)) {
     mrb_free(mrb, s1->as.heap.ptr);
   }
+
+  RSTR_UNSET_NOFREE_FLAG(s1);
+
+  if (RSTR_SHARED_P(s2)) {
+L_SHARE:
     RSTR_UNSET_EMBED_FLAG(s1);
     s1->as.heap.ptr = s2->as.heap.ptr;
     s1->as.heap.len = len;
@@ -1385,6 +1388,7 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
   }
   else {
     if (len <= RSTRING_EMBED_LEN_MAX) {
+      RSTR_UNSET_SHARED_FLAG(s1);
       RSTR_SET_EMBED_FLAG(s1);
       memcpy(s1->as.ary, RSTR_PTR(s2), len);
       RSTR_SET_EMBED_LEN(s1, len);

test/t/string.rb

@@ -320,6 +320,13 @@ assert('String#replace', '15.2.10.5.28') do
   b.replace(c);
   c.replace(b);
   assert_equal c, b
+
+  # shared string
+  s = "foo" * 100
+  a = s[10, 90]                # create shared string
+  assert_equal("", s.replace(""))    # clear
+  assert_equal("", s)          # s is cleared
+  assert_not_equal("", a)      # a should not be affected
 end
 
 assert('String#reverse', '15.2.10.5.29') do