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, 5 insertions, 6 deletions

diff --git a/mrblib/string.rb b/mrblib/string.rb
index 07b80b340..397603e9d 100644
--- a/mrblib/string.rb
+++ b/mrblib/string.rb
@@ -12,7 +12,7 @@ class String
   def each_line(rs = "\n", &block)
     return to_enum(:each_line, rs, &block) unless block
     return block.call(self) if rs.nil?
-    rs = rs.to_str
+    rs = rs.__to_str
     offset = 0
     rs_len = rs.length
     this = dup
@@ -67,7 +67,7 @@ class String
       block = nil
     end
     if !replace.nil? || !block
-      replace = replace.to_str
+      replace = replace.__to_str
     end
     offset = 0
     result = []
@@ -129,12 +129,12 @@ class String
     end
 
     pattern, replace = *args
-    pattern = pattern.to_str
+    pattern = pattern.__to_str
     if args.length == 2 && block
       block = nil
     end
     unless block
-      replace = replace.to_str
+      replace = replace.__to_str
     end
     result = []
     this = dup
@@ -245,14 +245,13 @@ class String
   ##
   # ISO 15.2.10.5.3
   def =~(re)
-    raise TypeError, "type mismatch: String given" if re.respond_to? :to_str
     re =~ self
   end
 
   ##
   # ISO 15.2.10.5.27
   def match(re, &block)
-    if re.respond_to? :to_str
+    if String === re
       if Object.const_defined?(:Regexp)
         r = Regexp.new(re)
         r.match(self, &block)