cmd/go: list modifies local go.sum even when package does not exist in queried module #36260
Description
What version of Go are you using (go version)?
$ go version go version go1.13.5 linux/amd64
Does this issue reproduce with the latest release?
Yes.
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="on" GOARCH="amd64" GOBIN="/home/user/bin" GOCACHE="/home/user/.cache/go-build" GOENV="/home/user/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/user" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/home/user/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="/home/user/tmp/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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build218352463=/tmp/go-build -gno-record-gcc-switches"
What did you do?
~ $ mkdir tmp
~ $ cd tmp
~/tmp $ go mod init module
go: creating new go.mod: module module
~/tmp $ stat go.sum
stat: cannot stat 'go.sum': No such file or directory
~/tmp $ go list golang.org/x/tools/nonexistent
go: finding golang.org/x/tools latest
can't load package: package golang.org/x/tools/nonexistent: module golang.org/x/tools@latest found (v0.0.0-20191223235410-3721262b3e7c), but does not contain package golang.org/x/tools/nonexistent
~/tmp $ cat go.sum
golang.org/x/tools v0.0.0-20191223235410-3721262b3e7c h1:PeFrxQ8YTAKg53UR8aP/nxa82lQYIdb+pd1bfg3dBDM=
What did you expect to see?
No change to go.sum.
What did you see instead?
An addition to go.sum.