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

$ go version
go version go1.17.1 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/niaow/.cache/go-build"
GOENV="/home/niaow/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/niaow/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/niaow/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.1"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build924292266=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I used bufio.NewWriter to implement the io.WriterTo interface, passing in another bufio.Writer.

https://play.golang.org/p/iBz8rYK1Uae

What did you expect to see?

As defined in the documentation for bufio.NewWriter:

NewWriter returns a new Writer whose buffer has the default size.

The bufio.NewWriter call inside the WriterTo implementation returns a new bufio.Writer, and the WriterTo implementation correctly calculates that it wrote 2 bytes to w.

What did you see instead?

The bufio.NewWriter call returns the input writer, causing the WriterTo implementation to calculate (incorrectly) that it wrote -4094 bytes when handling an error.

This behavior is properly defined on bufio.NewWriterSize, but not bufio.NewWriter.