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

$ go version
go version go1.14.6 linux/amd64

Does this issue reproduce with the latest release?

Yes, go1.14.6.

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/me/.cache/go-build"
GOENV="/home/me/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/me/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"
GCCGO="gccgo"
AR="ar"
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-build878029847=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Preamble: Sorry if this is known behavior for the regexp package. It was not obvious to me after reading the documentation.

I provided regexp.MatchReader with a RuneReader that always returns a non-EOF error (https://play.golang.org/p/_CVP4uouxy4):

package main

import (
	"errors"
	"fmt"
	"regexp"
)

var notEOF = errors.New("not EOF")

type notEOFErrRuneReader struct{}

func (notEOFErrRuneReader) ReadRune() (rune, int, error) {
	return 0, 0, notEOF
}

func main() {
	matched, err := regexp.MatchReader(".+", notEOFErrRuneReader{})
	fmt.Println(matched)
	fmt.Println(err)
}

What did you expect to see?

I expected regexp.MatchReader to return a non-nil error because the underlying RuneReader had an error.

What did you see instead?

regexp.MatchReader returned false (no match) and a nil error.

I also noticed the *Regexp methods that take a RuneReader do not return errors. I doubt much can be done about that other than potentially some documentation improvements.

I'm not sure if anything should be done about regexp.MatchReader but since the signature has an error it may be surprising that it only returns an error if the provided regular expression does not compile, not if the RuneReader has an error. If the behavior cannot be changed it might be good to have some documentation to let people know regexp.MatchReader may return false because all of the input was not available due to an error.

I was able to work around the issue by wrapping the rune reader and snooping on the results (https://play.golang.org/p/X-u5lkZ6hXw).