cmd/go: no "module requires Go 1.nn" warning when a package is not found

[bradfitz]

Tested with both Go 1.17.x and Go 1.18(.0)...

If you have a module with a go 1.n directive and try to build with older Go, you get a nice error message ("note: module requires Go 1.18") from cmd/compile when the build fails. This is good:

$ cat go.mod; echo --; cat *.go
module new-symbol

go 1.18
--
package main

import "unicode/utf8"

func main() {
        _ = utf8.AppendRune
}
$ go version
go version go1.18 linux/amd64
$ go run .
$ ~/sdk/go1.17/bin/go run .
# new-symbol
./new-symbol.go:6:6: undefined: utf8.AppendRune
note: module requires Go 1.18

But if the the project is using a new package (say, net/netip in a go 1.18 module) and then a Go 1.17 user tries to build:

$ cat go.mod; echo --; cat *.go
module new-pkg

go 1.18
--
package main

import _ "net/netip"

func main() {
}
pkg$ go version
go version go1.18 linux/amd64
$ go run .
$ ~/sdk/go1.17/bin/go run .
new-pkg.go:3:8: package net/netip is not in GOROOT (/home/bradfitz/sdk/go1.17/src/net/netip)

... they don't get the nice "note: module requires Go 1.18" hint.

Instead they just think you broke the build and file bug reports against you, even though your go.mod says 1.18.

I verified that this bug is still present in 1.18(.0) too:

$ cat go.mod; echo --; cat *.go
module new-pkg

go 1.19
--
package main

import _ "some/future"

func main() {
}
$ go version
go version go1.18 linux/amd64
$ go run .
new-pkg.go:3:8: package some/future is not in GOROOT (/home/bradfitz/sdk/go1.18/src/some/future)

Filed on request of @ianlancetaylor who realized that it was cmd/go that's missing the error.

/cc @bcmills @matloob @nickoneill @josharian

[bcmills]

Duplicate of #48966