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

$ go version
go version go1.17.1 linux/amd64

Does this issue reproduce with the latest release?

Yes, even with the latest gotip version.

$ gotip version
go version devel go1.18-4847c47 Wed Sep 15 03:29:46 2021 +0000 linux/amd64
$ gotip get .
example imports
        github.com/golang-migrate/migrate/v4/source/iofs: cannot find module providing package github.com/golang-migrate/migrate/v4/source/iofs

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/anli/.cache/go-build"
GOENV="/home/anli/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/anli/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/anli/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.1"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/tmp/tmp.oODGF8c7us/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-build1486045652=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I wanted to use embedded SQL migration files in a Go 1.17 project and this required to use the github.com/golang-migrate/migrate/v4/source/iofs migration source driver. The source/iofs driver was added to golang-migrate/migrate shortly after Go 1.16 with embed.FS was released. In order to be compatible with Go versions prior 1.16 a build tag was added such that source/iofs can only be used with Go 1.16 and newer. The documentation about build constraints states:

• a term for each Go major release, through the current version:
  "go1.1" from Go version 1.1 onward, "go1.12" from Go 1.12, and so on.

If this holds then the package must be importable in Go 1.17 projects as well, but it isn't. A minimal example that demonstrates this issue can be found in this playground. Here is the example's source code:

package main

import (
	"database/sql"
	"embed"

	"github.com/golang-migrate/migrate/v4"
	"github.com/golang-migrate/migrate/v4/database/postgres"
	"github.com/golang-migrate/migrate/v4/source/iofs"
	_ "github.com/lib/pq"
)

//go:embed *
var migrationFS embed.FS

func main() {
	db, _ := sql.Open("postgres", "postgres://localhost:5432/database?sslmode=enable")
	driver, _ := postgres.WithInstance(db, &postgres.Config{})
	src, _ := iofs.New(migrationFS, ".")
	m, _ := migrate.NewWithInstance("iofs", src, "postgres", driver)
}

Interestingly, the package can be imported when go1.17 was added to the build tags as well. I did this in golang-migrate/migrate#618 .

What did you expect to see?

That github.com/golang-migrate/migrate/v4/source/iofs can be imported in a Go 1.17 project since the build tag //go:build go1.16 allows all Go version starting from 1.16.

What did you see instead?

That go get refuses to add the import:

$ go get .
cmd/inject/inject.go:30:2: no required module provides package github.com/golang-migrate/migrate/v4/source/iofs; to add it:
        go get github.com/golang-migrate/migrate/v4/source/iofs