Remove implicit conversion using `to_str` method; fix #3854

We have added internal convenience method `__to_str` which
does string type check.

The issue #3854 was fixed but fundamental flaw of lack of stack
depth check along with fibers still remains. Use `MRB_GC_FIXED_ARENA`
for workaround.

1 files changed, 9 insertions, 2 deletions

diff --git a/src/class.c b/src/class.c
index 50ab0ea59..90c73104e 100644
--- a/src/class.c
+++ b/src/class.c
@@ -504,10 +504,17 @@ check_type(mrb_state *mrb, mrb_value val, enum mrb_vtype t, const char *c, const
   return tmp;
 }
 
+#define CHECK_TYPE(mrb, val, t, c) do { \
+  if (mrb_type(val) != (t)) {\
+    mrb_raisef(mrb, E_TYPE_ERROR, "expected %S", mrb_str_new_lit(mrb, c));\
+  }\
+} while (0)
+
 static mrb_value
 to_str(mrb_state *mrb, mrb_value val)
 {
-  return check_type(mrb, val, MRB_TT_STRING, "String", "to_str");
+  CHECK_TYPE(mrb, val, MRB_TT_STRING, "String");
+  return val;
 }
 
 static mrb_value
@@ -1972,7 +1979,7 @@ mrb_mod_const_get(mrb_state *mrb, mrb_value mod)
   }
 
   /* const get with class path string */
-  path = mrb_string_type(mrb, path);
+  path = mrb_ensure_string_type(mrb, path);
   ptr = RSTRING_PTR(path);
   len = RSTRING_LEN(path);
   off = 0;