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

Go1.19-beta1

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

Open the 1.19 beta 1 changelog and go to the io section, then read:

MultiReader's result now implements WriterTo unconditionally. If any underlying reader does not implement WriterTo, it is simulated appropriately.

However the https://pkg.go.dev/io#MultiReader documentation does not include that:

// MultiReader returns a Reader that's the logical concatenation of
// the provided input readers. They're read sequentially. Once all
// inputs have returned EOF, Read will return EOF.  If any of the readers
// return a non-nil, non-EOF error, Read will return that error.

What did you expect to see?

A sentence including the WriterTo behaviour.

What did you see instead?

Nothing

Backstory

I didn't included a note of that while implementing WriterTo on MultiReader since I thought this was implementation details (I see WriterTo like an implicit optional extension of all Reader interfaces) and this is the behaviour you expect (you can find a youtube video where a member of the go team do a live codding session and talks about zero copying on multireaders which was not a thing back then).

If it is changelog worthy then it should be also documented in the code, so future developers can learn about it.