Fix ActiveSupport::Multibyte::Chars#slice for empty strings when starting offset is negative [#4717 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

2 files changed, 3 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/multibyte/chars.rb b/activesupport/lib/active_support/multibyte/chars.rb
index 1134d1ccc6..9251b26f62 100644
--- a/activesupport/lib/active_support/multibyte/chars.rb
+++ b/activesupport/lib/active_support/multibyte/chars.rb
@@ -318,7 +318,8 @@ module ActiveSupport #:nodoc:
           character = Unicode.u_unpack(@wrapped_string)[args[0]]
           result = character.nil? ? nil : [character].pack('U')
         else
-          result = Unicode.u_unpack(@wrapped_string).slice(*args).pack('U*')
+          cps = Unicode.u_unpack(@wrapped_string).slice(*args)
+          result = cps.nil? ? nil : cps.pack('U*')
         end
         result.nil? ? nil : chars(result)
       end
diff --git a/activesupport/test/multibyte_chars_test.rb b/activesupport/test/multibyte_chars_test.rb
index 66aa22ec20..610295fa89 100644
--- a/activesupport/test/multibyte_chars_test.rb
+++ b/activesupport/test/multibyte_chars_test.rb
@@ -397,6 +397,7 @@ class MultibyteCharsUTF8BehaviourTest < Test::Unit::TestCase
     assert_equal 'こ', @chars.slice(0)
     assert_equal 'わ', @chars.slice(3)
     assert_equal nil, ''.mb_chars.slice(-1..1)
+    assert_equal nil, ''.mb_chars.slice(-1, 1)
     assert_equal '', ''.mb_chars.slice(0..10)
     assert_equal 'にちわ', @chars.slice(1..3)
     assert_equal 'にちわ', @chars.slice(1, 3)