What version of Go are you using (go version)?

1.16.6

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using?

linux/amd64

What did you do?

Given this string: "Ꮖ-Ꮩ-Ꭺ-Ꮹ-Ꭼ-Ꮮ", convert it to lower case via x/text/cases.Fold().Bytes(...). This produces a valid but unexpected result (and many common fonts don't contain characters for the output).

What did you expect to see?

I expected the characters to go through unchanged. That's because x/text/cases claims to comply with Unicode version 13. ICU (I tested v63.2 and v66.1), which include Unicode 13 and below, leave the text unchanged when passed to ICU's u_strFoldCase.