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

1.12

Does this issue reproduce with the latest release?

N/A

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

Ubuntu 18.04

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/[home]/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/[home]/go"
GOPROXY=""
GORACE=""
GOROOT="/home/[home]/.go"
GOTMPDIR=""
GOTOOLDIR="/home/[home]/.go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build210454728=/tmp/go-build -gno-record-gcc-switches"

What did you do?

func TestMimeBoundary(t *testing.T) {
	for _, b := range []string{"", " boundary="} {
		r := http.Request{}
		r.Header = make(http.Header)
		r.Header.Set("Content-Type", "multipart/form-data;" + b)
		_, err := r.MultipartReader()
		fmt.Printf("%T %v\n", err, err)
	}
}

Results in:

*http.ProtocolError no multipart boundary param in Content-Type
*http.ProtocolError request Content-Type isn't multipart/form-data

This means comparing errors against http.ErrNotMultipart is not reliable.

What did you expect to see?

I would expect an error that doesn't assert that data with the content type multipart/form-data is not multipart/form-data. More abstractly, I would expect a set of errors to compare against such that when the error is http.ErrNotMultipart it is reliable to proceed assuming the client did not intend the http body to be multipart rather than they just bolloxed it up.

What did you see instead?

As above.