cmd/go: do not permit importing a main package

Fixes #4210.

Change-Id: Id981814a6e55a57403ce7a8ac45ab3ba081a3a86
Reviewed-on: https://go-review.googlesource.com/10925Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -1830,3 +1830,21 @@ func TestGoGetRscIoToolstash(t *testing.T) {
 	tg.cd(tg.path("src/rsc.io"))
 	tg.run("get", "./toolstash")
 }
+
+// Test that you can not import a main package.
+func TestIssue4210(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.tempFile("src/x/main.go", `package main
+		var X int
+		func main() {}`)
+	tg.tempFile("src/y/main.go", `package main
+		import "fmt"
+		import xmain "x"
+		func main() {
+			fmt.Println(xmain.X)
+		}`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.runFail("build", "y")
+	tg.grepBoth("is a program", `did not find expected error message ("is a program")`)
+}

src/cmd/go/pkg.go

@@ -631,6 +631,16 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 			continue
 		}
 		p1 := loadImport(path, p.Dir, stk, p.build.ImportPos[path])
+		if p1.Name == "main" {
+			p.Error = &PackageError{
+				ImportStack: stk.copy(),
+				Err:         fmt.Sprintf("import %q is a program, not an importable package", path),
+			}
+			pos := p.build.ImportPos[path]
+			if len(pos) > 0 {
+				p.Error.Pos = pos[0].String()
+			}
+		}
 		if p1.local {
 			if !p.local && p.Error == nil {
 				p.Error = &PackageError{