This result is consistent since the day that @ianlancetaylor introduces os.IsTimeout in Go 1.10.
However iotest.ErrTimeout lives way longer than io.IsTimeout.

The question is whether the Go team will recognize this as expected behavior with more documentation or a bug that should be fixed.

I think that iotest.TimeoutReader is just useless for writing I/O tests if iotest.ErrTimeout isn't matched by os.IsTimeout.

Making it match is just switching ErrTimeout from being built from errors.New to using a specific type that has a Timeout method.

type errTimeout struct{}

func (errTimeout) Error() string {
    return "timeout"
}

func (errTimeout) Timeout() bool {
    return true
}

var ErrTimeout error = errTimeout{}

I can provide this as a PR (with added documentation).

Also /cc-ing @mpvl @josharian as owners of testing

Just for some context, os.ErrTimeout was removed in #33411. There's also some discussion that seems related in #30322.

os.IsTimeout does what it does and won't be changing. The comments need to be updated to mark it as deprecated - use errors.Is instead. Fixing the doc comments is #41122 .