[glhaynes]

If the fs detects a bit error does it flag the file as entirely unreadable? Move it to lost+found? Force me to restore the file from a backup?

For files stored in iCloud Drive, if that version of the file exists in the cloud, the OS could automatically re-fetch the file. But, yeah, for lots of circumstances there's not going to be a "good" option to give the user.

EDIT: Same applies to Time Machine (or whatever Apple's backup solution will be called in the APFS era).

[mindajar]

It was a stealthy feature addition that went totally unannounced, but as of 10.11, Time Machine stores file checksums in the backup. See 'tmutil verifyChecksums'.