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

$ go version
go version go1.13 darwin/amd64

Does this issue reproduce with the latest release?

Same behavior in master.
go version devel +78a3734714 Tue Sep 24 21:33:10 2019 +0000 darwin/amd64

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/aaronb/Library/Caches/go-build"
GOENV="/Users/aaronb/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/aaronb/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/aaronb/go/src/arista/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/dh/8tf0gx091zs_02mw_s0c956w0000gp/T/go-build130633312=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Ran go mod init on a repository with a Gopkg.toml/Gopkg.lock generated by dep.
This Gopkg.toml contains an alternative source for an import, which translates to a replace in go.mod.

The intention is that code imports a dependency's original path. The replace is then used to use an alternative repo that contains an additional feature/bug fix that is waiting to merge upstream.

Reproduced here:
https://github.com/aaronbee/replace_repro

What did you expect to see?

After running go mod init, go.mod should be valid:

require upstream_repo v0.0.0

replace upstream_repo => alternate_repo ALT_REVISION

In this case the upstream_path is golang.org/x/tools and alternate_repo is github.com/aaronbee/tools

What did you see instead?

go.mod:

require alternate_repo ALT_REVSION

replace upstream_repo ALT_REVISION => alternate_repo ALT_REVISION

Actual go.mod.

This results in build errors because ALT_REVSION is not present in upstream_repo. It is only valid in alternate_repo.

$ go build
go: finding golang.org/x/tools 5cb9115d21e89da8e75c5b207bfa1c512c315161
go: finding golang.org/x/tools 5cb9115d21e89da8e75c5b207bfa1c512c315161
go: errors parsing go.mod:
/Users/aaronb/go/src/github.com/aaronbee/replace_repro/go.mod:5: replace golang.org/x/tools: version "5cb9115d21e89da8e75c5b207bfa1c512c315161" invalid: unknown revision 5cb9115d21e89da8e75c5b207bfa1c512c315161