I intentionally add non-ASCII characters to our code, so that an incorrectly configured IDE or bad tool fails.

75% of the development team has at least one non-ASCII character in their name, so it would be pretty rude otherwise.

It's much better to knowingly reject a tool at the start, since it can't handle ordinary characters, than find out a year later with the first e.g. British customer that it can't handle "£".

If you don't allow non-ascii then you may have to mangle some people's names in the copyright header.

It's good to check files for unexpected characters, though. Here's some Perl to do it:

  perl -e 'binmode(STDIN, ":utf8"); binmode(STDOUT, ":utf8");
  foreach (split(//, join("", <STDIN>))) { ++$c{$_}; }
  foreach (sort(keys %c)) { printf "%8d %s\n", $c{$_}, $_; }'

I tested that on a text file I was working on ... and I discovered that the file contained a BOM (U+FEFF), not at the start of the file, but at a random point in the middle of the file. I've deleted it. Who knows what problems it might have caused for me later?

You could have a pre-commit git hook that refers to a whitelist of allowed non-ascii characters, or also allow all alphabetic characters, or something like that.

It should be checked as part of the CI too, if you're doing that, some people might never install the hooks, and some people might git commit --no-verify.

This issue would never have happened using modern software. And if your text (including source code) still isn't in UTF-8, you're doing it wrong.

(I guess unless you're using some specific, very performance-conscious system that has to use 7/8-bit characters and is never going to connect to the Internet anyway.)

It's like preventing the headache with a guillotine.