env GO111MODULE=on

# go.sum should list directly used modules and dependencies
go get rsc.io/quote@v1.5.2
go mod -sync
grep rsc.io/sampler go.sum

# go.sum should not normally lose old entries
go get rsc.io/quote@v1.0.0
grep 'rsc.io/quote v1.0.0' go.sum
grep 'rsc.io/quote v1.5.2' go.sum
grep rsc.io/sampler go.sum

# go mod -sync should clear dead entries from go.sum
go mod -sync
grep 'rsc.io/quote v1.0.0' go.sum
! grep 'rsc.io/quote v1.5.2' go.sum
! grep rsc.io/sampler go.sum

# go.sum with no entries is OK to keep
# (better for version control not to delete and recreate.)
cp x.go.noimports x.go
go mod -sync
exists go.sum
! grep . go.sum

-- go.mod --
module x
-- x.go --
package x
import _ "rsc.io/quote"
-- x.go.noimports --
package x