Simplify arguments check in `String#index`

Also fix document about type of the first argument.

1 files changed, 4 insertions, 0 deletions

diff --git a/test/t/string.rb b/test/t/string.rb
index c820bfa92..01f3da327 100644
--- a/test/t/string.rb
+++ b/test/t/string.rb
@@ -451,12 +451,16 @@ assert('String#index', '15.2.10.5.22') do
   assert_equal 3, 'abcabc'.index('a', 1)
   assert_equal 5, "hello".index("", 5)
   assert_equal nil, "hello".index("", 6)
+  assert_equal 3, "hello".index("l", -2)
+  assert_raise(ArgumentError) { "hello".index }
+  assert_raise(TypeError) { "hello".index(101) }
 end
 
 assert('String#index(UTF-8)', '15.2.10.5.22') do
   assert_equal 0, '⓿➊➋➌➍➎'.index('⓿')
   assert_nil '⓿➊➋➌➍➎'.index('➓')
   assert_equal 6, '⓿➊➋➌➍➎⓿➊➋➌➍➎'.index('⓿', 1)
+  assert_equal 6, '⓿➊➋➌➍➎⓿➊➋➌➍➎'.index('⓿', -7)
   assert_equal 6, "⓿➊➋➌➍➎".index("", 6)
   assert_equal nil, "⓿➊➋➌➍➎".index("", 7)
   assert_equal 0, '⓿➊➋➌➍➎'.index("\xe2")