cmd/go: unresolved import masks vet warnings in 'go test' and 'go vet'

[CzBiX]

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

$ go version
go version go1.15.6 linux/amd64

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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build241876888=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ find
.
./foo
./foo/bar.go
$ cat foo/bar.go
// +build windows
package foo

import _ "golang.org/x/sys/windows"

$ go test ./...
package ./foo
        imports golang.org/x/sys/windows: build constraints exclude all Go files in /go/src/golang.org/x/sys/windows

What did you expect to see?

test success, test ignore files with tags.

What did you see instead?

test failed.

[bcmills]

Your // +build windows comment has no newline after it, so it is interpreted as a package comment instead of a build constraint.

There is a vet check for that mistake, and go test runs that vet check implicitly. However, it appears that in this case the build error is masking the clearer vet warning.

CC @jayconrod @matloob

[bcmills]

This is somewhat related to #37803.

[CzBiX]

Thanks.
BTW, go vet ./... got the same error.