bufio: reject UnreadByte or UnreadRune after a Discard or WriteTo
Discard is not really a read operation, and in theory it could
Seek the underlying Reader without actually reading anything,
so an UnreadByte following a Discard is disallowed.
Similarly, although WriteTo usually does end up calling Read on the
underlying buffer, if the underlying Reader implements io.WriterTo it
may instead terminate in a call to WriteTo, without ever buffering or
even seeing the last byte written. (It is conceptually read-like, but
not strictly “a read operation”.)
Fixes #48446
Change-Id: Ide6f2b157332b423486810399f66140c914144e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/351810
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Joe Tsai <joetsai@digital-static.net> Reviewed-by: Joe Tsai <joetsai@digital-static.net>