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

$ go version
1.11.2

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/gengbing/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/gengbing/Workspace/golang"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.2/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.2/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/gengbing/Workspace/tmp/gomodbug/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 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/z5/hn4wg1jd1pb5z2wl3x8y9ng40000gn/T/go-build233013232=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

mkdir gomodbug
cd gomodbug

cat << EOF > go.mod
module gomodbug

replace (
	github.com/mailru/easyjson => github.com/edwingeng/easyjson v0.0.0-20180909112517-9ee4e6945130
)
EOF

cat << EOF > stub.go
// +build untouchable

package gomodbug

import (
        _ "github.com/mailru/easyjson/easyjson"
)
EOF

cat << EOF > x.go
package gomodbug

type Foo struct {
	A int
	B string
}
EOF

go install github.com/mailru/easyjson/easyjson
easyjson -omit_empty -all x.go

echo
cat go.sum
echo

go mod tidy
cat go.sum

What did you expect to see?

'go mod tidy' should not change go.sum.

What did you see instead?

The 'cat go.sum' before 'go mod tidy' outputs:

github.com/edwingeng/easyjson v0.0.0-20180909112517-9ee4e6945130 h1:Ke1vi7wBhQv5uramtBfIIJ00crorqtiDAdcqzfJ3/B0=
github.com/edwingeng/easyjson v0.0.0-20180909112517-9ee4e6945130/go.mod h1:8Db6a8b4MGQg08ypk+IK5qQDIGPdVGGp+9RlA1FFRnE=

The 'cat go.sum' after 'go mod tidy' outputs:

github.com/edwingeng/easyjson v0.0.0-20180909112517-9ee4e6945130/go.mod h1:8Db6a8b4MGQg08ypk+IK5qQDIGPdVGGp+9RlA1FFRnE=