cmd/go: clarify error when package is removed in a module

If no module in the build list provides an imported package, we
try to upgrade to the "@latest" version. If there is a requirement on
a version of the module which is newer than the "@latest" version
(e.g., a prerelease or pseudoversion), we cannot upgrade further.

We previously reported "looping trying to add package" when we saw the
package in "@latest" but it was removed later. The meaning of this is
unclear for users, so with this change, we explain the package was
removed.

Fixes #30394

Change-Id: I1b7fec2c37e762fb600e66ee8a4df4aeaf13e67a
Reviewed-on: https://go-review.googlesource.com/c/go/+/169720
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/import.go

@@ -28,6 +28,10 @@ import (
 type ImportMissingError struct {
 	ImportPath string
 	Module     module.Version
+
+	// newMissingVersion is set to a newer version of Module if one is present
+	// in the build list. When set, we can't automatically upgrade.
+	newMissingVersion string
 }
 
 func (e *ImportMissingError) Error() string {
@@ -189,7 +193,18 @@ func Import(path string) (m module.Version, dir string, err error) {
 		}
 		return module.Version{}, "", &ImportMissingError{ImportPath: path}
 	}
-	return m, "", &ImportMissingError{ImportPath: path, Module: m}
+	newMissingVersion := ""
+	for _, bm := range buildList {
+		if bm.Path == m.Path && semver.Compare(bm.Version, m.Version) > 0 {
+			// This typically happens when a package is present at the "@latest"
+			// version (e.g., v1.0.0) of a module, but we have a newer version
+			// of the same module in the build list (e.g., v1.0.1-beta), and
+			// the package is not present there.
+			newMissingVersion = bm.Version
+			break
+		}
+	}
+	return m, "", &ImportMissingError{ImportPath: path, Module: m, newMissingVersion: newMissingVersion}
 }
 
 // maybeInModule reports whether, syntactically,

src/cmd/go/internal/modload/load.go

@@ -547,6 +547,9 @@ func (ld *loader) load(roots func() []string) {
 		}
 		for _, pkg := range ld.pkgs {
 			if err, ok := pkg.err.(*ImportMissingError); ok && err.Module.Path != "" {
+				if err.newMissingVersion != "" {
+					base.Fatalf("go: %s: package provided by %s at latest version %s but not at required version %s", pkg.stackText(), err.Module.Path, err.Module.Version, err.newMissingVersion)
+				}
 				if added[pkg.path] {
 					base.Fatalf("go: %s: looping trying to add package", pkg.stackText())
 				}

src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.0.txt

+The deprecated package is present in this version (which is @latest) but
+is deleted in a newer prerelease version.
+
+-- .mod --
+module example.com/missingpkg
+-- .info --
+{"Version":"v1.0.0"}
+-- lib.go --
+package lib
+-- deprecated/deprecated.go --
+package deprecated

src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.1-beta.txt

+The deprecated package is deleted in this version.
+
+-- .mod --
+module example.com/missingpkg
+-- .info --
+{"Version":"v1.0.1-beta"}
+-- lib.go --
+package lib

src/cmd/go/testdata/mod/example.com_usemissingpre_v1.0.0.txt

+This module requires example.com/missingpkg at a prerelease version, which
+is newer than @latest.
+
+-- .mod --
+module example.com/usemissingpre
+
+require example.com/missingpkg v1.0.1-beta
+-- .info --
+{"Version":"v1.0.0"}
+-- use.go --
+package use
+
+import _ "example.com/missingpkg"

src/cmd/go/testdata/script/mod_missingpkg_prerelease.txt

+env GO111MODULE=on
+
+! go list use.go
+stderr 'import "example.com/missingpkg/deprecated": package provided by example.com/missingpkg at latest version v1.0.0 but not at required version v1.0.1-beta'
+
+-- use.go --
+package use
+
+import (
+	_ "example.com/missingpkg/deprecated"
+	_ "example.com/usemissingpre"
+)