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

$ go version
go version go1.21rc3 darwin/arm64

Does this issue reproduce with the latest release?

No(-ish).

I first ran into this in a module whose 'go' line is 1.21, so the latest release today (1.20.6) isn't applicable.

If I change the 'go' line to 1.20, then 'go mod tidy' no longer deletes as many go.sum lines, and the issue doesn't reproduce.

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

$ go env
GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/gopher/Library/Caches/go-build'
GOENV='/Users/gopher/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='loopvar'
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/gopher/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/gopher/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21rc3'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_0/h0671fcn4rgb5pn9c745dx2h0000gn/T/go-build1549452710=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I ran go mod tidy in a medium sized local module whose go.mod has 'go 1.21' (primarily to help test this language version and its semantics, not because I need it; this go.mod file isn't public).

Then I ran go list -m -u all.

Edit: It turned out trivial to trigger the issue with a very minified top-level go.mod file. Here's a complete reproducer:

$ cd $(mktemp -d)
$ go mod init issue61605.test
$ echo 'package p; import _ "golang.org/x/build/maintner/reclog"' > p.go
$ go get golang.org/x/build@v0.0.0-20230726111318-84c18c56e0b0
$ go mod tidy
$ go list -m -u all

What did you expect to see?

The usual list of modules and available updates, and exit code 0.

What did you see instead?

$ go list -m -u all
go: updates to go.sum needed, disabled by -mod=readonly
$ echo $?
1

Running the go list command with -mod=mod re-adds the go.sum lines and makes the command work.

Or setting go to 1.20 instead of 1.21 and re-running go mod tidy also works around the issue.