cmd/go: mod edit -fmt reports latest as invalid version

[myitcv]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPROXY=""
GORACE=""
GOROOT="/home/myitcv/gos"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/myitcv/gostuff/src/github.com/myitcv/gobin/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-build194100036=/tmp/go-build -gno-record-gcc-switches"

What did you do?

export GOPATH=$(mktemp -d)
export PATH=$GOPATH/bin:$PATH
cd $(mktemp -d)
mkdir hello
cd hello
go mod init example.com/hello
go mod edit -require=example.com/goodbye@latest
go mod edit -fmt 

What did you expect to see?

A zero exit code.

What did you see instead?

A non-zero exit code and:

$ go mod edit -fmt
go: errors parsing go.mod:
/tmp/tmp.OkrAZeigOA/hello/go.mod:3: invalid module version "latest": version must be of the form v1.2.3

[bcmills]

Duplicate of #27060.

(Resolution per @rsc: “if you want to add a requirement with all the semantic extras, use 'go get'.”)

[bcmills]

Wait, no. This is the inverse problem: we're validating something we shouldn't be.