Callback should yield with pattern every time

mrblib/string.rb

@@ -59,16 +59,26 @@ class String
   # ISO 15.2.10.5.18
   def gsub(*args, &block)
     return to_enum(:gsub, *args) if args.length == 1 && !block
-    if args.size == 2
+    raise ArgumentError, "wrong number of arguments" unless (1..2).include?(args.length)
+
     pattern, replace = *args
     plen = pattern.length
+    if args.length == 2 && block
+      block = nil
+    end
+    if !replace.nil? || !block
       replace = replace.to_str
+    end
     offset = 0
     result = []
     while found = index(pattern, offset)
       result << self[offset, found - offset]
       offset = found + plen
-        result << replace.__sub_replace(self[0, found], pattern, self[offset..-1] || "")
+      result << if block
+        block.call(pattern).to_s
+      else
+        replace.__sub_replace(self[0, found], pattern, self[offset..-1] || "")
+      end
       if plen == 0
         result << self[offset, 1]
         offset += 1
@@ -76,11 +86,6 @@ class String
     end
     result << self[offset..-1] if offset < length
     result.join
-    elsif args.size == 1 && block
-      split(args[0], -1).join(block.call(args[0]))
-    else
-      raise ArgumentError, "wrong number of arguments"
-    end
   end
 
   ##

test/t/string.rb

@@ -373,6 +373,11 @@ assert('String#gsub', '15.2.10.5.18') do
   assert_equal('A',      'a'.gsub('a'){|w| w.capitalize })
   assert_equal("<a><><>", 'a'.gsub('a', '<\0><\1><\2>'))
   assert_equal(".h.e.l.l.o.", "hello".gsub("", "."))
+  a = []
+  assert_equal(".h.e.l.l.o.", "hello".gsub("") { |i| a << i; "." })
+  assert_equal(["", "", "", "", "", ""], a)
+  assert_raise(ArgumentError) { "".gsub }
+  assert_raise(ArgumentError) { "".gsub("", "", "") }
 end
 
 assert('String#gsub with backslash') do