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

$ go version
go version go1.12 linux/amd64

Does this issue reproduce with the latest release?

I have the latest 1.12

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

Linux Mint 19.1

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/toshiba/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/toshiba/Documents/workspace/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="0"
GOMOD="/home/toshiba/Documents/workspace/go_projects/active-sport-server/go.mod"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build887106064=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Tried to run GO111MODULE=on CGO_ENABLED=0 go build outside of $GOPATH in order to try go modules. But there seems some problem with net/http package.

$ cat main.go 
package main

import "net/http"

func main() {
	http.ListenAndServe(":8080", nil)
}

What did you expect to see?

correctly built

What did you see instead?

$ GO111MODULE=on CGO_ENABLED=0 go build
# crypto/rc4
/usr/local/go/src/crypto/rc4/rc4_asm.go:15:18: (*Cipher).XORKeyStream redeclared in this block
	previous declaration at /usr/local/go/src/crypto/rc4/rc4.go:61:6
# bytes
/usr/local/go/src/bytes/bytes_decl.go:10:6: IndexByte redeclared in this block
	previous declaration at /usr/local/go/src/bytes/bytes.go:101:34
/usr/local/go/src/bytes/bytes_decl.go:17:6: Equal redeclared in this block
	previous declaration at /usr/local/go/src/bytes/bytes.go:18:25
/usr/local/go/src/bytes/bytes_decl.go:24:6: Compare redeclared in this block
	previous declaration at /usr/local/go/src/bytes/bytes.go:37:27
# crypto/cipher
/usr/local/go/src/crypto/cipher/xor_amd64.go:9:6: xorBytes redeclared in this block
	previous declaration at /usr/local/go/src/crypto/cipher/xor.go:58:33
/usr/local/go/src/crypto/cipher/xor_amd64.go:22:6: xorWords redeclared in this block
	previous declaration at /usr/local/go/src/crypto/cipher/xor.go:83:27
# strings
/usr/local/go/src/strings/strings_decl.go:8:6: IndexByte redeclared in this block
	previous declaration at /usr/local/go/src/strings/strings.go:150:34