cmd/go: can't build go1.10 from source using go1.12

[matloob]

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

Go 1.12

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
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/matloob/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/matloob/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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/f4/9p58ddnj40x58zchdb36p7lr004_sl/T/go-build724269079=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I have Go 1.12 installed on my system. I cloned the go repo and checked out the tag go1.10.8.
Then i cd'd to the go repo's src directory and ran make.bash

What did you expect to see?

a successful build

What did you see instead?

matloob-macbookpro2:src matloob$ ./make.bash
Building Go cmd/dist using /usr/local/go.
go: cannot determine module path for source directory /Users/matloob/go110 (outside GOPATH, no import comments)

But make.bash does work when GO111MODULE is unset

[josharian]

@bcmills

[rsc]

We should fix this. We want old go trees to keep building with new ones as bootstrap. cmd/dist is a single-directory main package binary.

[bcmills]

I think this ties closely to #32027: we're erroring out because we do not have a module path (and therefore do not have a package path) for package in the working directory.

In theory we could proceed anyway: since the package in the current directory imports only standard-library packages, the only effects of its go.mod file would be to select the go version in use and set accurate debug information, and for most binaries those don't matter much.