Incorrect results of Sting#index and String#rindex.

[arcage]

Hi,

It seem the Sting#index and String#rindex return incorrect results, in case that searches an ascii Char in the string that includes multibyte letters.

My environment:

• Crystal 0.19.4 (2016-10-21)
• macOS sierra 10.12.1

Code:

# the both of the following results should be 3.
p "あいう_えお".index('_')
#=> nil
p "あいう_えお".rindex('_')
#=> nil

When String#index or String#rindex is called with the search argument that is an ascii Char, it delegates the process to #to_slice.

But, index of string is not equal with it of string.to_slice when that string includes multibyte letters.

This causes the incorrect results of File.basename and File.extname.

Those methods are calling String#rindex with '.' or File::SEPARETOR(/ or '\\').

File.basename("日本語/日本語ファイル.txt")
#=> "ル.txt"　(it shoud be "日本語ファイル.txt")

File.extname("日本語/日本語ファイル.txt")
#=> "" (it should be ".txt")

[asterite]

@arcage Thank you, you are totally correct! I applied this optimization recently but it was wrong, I was missing checking that the string is also made of ascii characters.