Check method existence in `to_enum'; ref #3773

mrbgems/mruby-enum-lazy/mrblib/lazy.rb

@@ -43,6 +43,9 @@ class Enumerator
     end
 
     def to_enum(meth=:each, *args, &block)
+      unless self.respond_to?(meth)
+        raise NoMethodError, "undefined method #{meth}"
+      end
       lz = Lazy.new(self, &block)
       lz.obj = self
       lz.meth = meth

mrbgems/mruby-enumerator/mrblib/enumerator.rb

@@ -115,6 +115,9 @@ class Enumerator
     else
       raise ArgumentError unless obj
     end
+    if @obj and !self.respond_to?(meth)
+      raise NoMethodError, "undefined method #{meth}"
+    end
 
     @obj = obj
     @meth = meth