cmd/go: fix missing internal import error

Fixes #11331.

Change-Id: I19b8172421044c301bc136fc8f7bfdadbf880e25
Reviewed-on: https://go-review.googlesource.com/12450
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/go_test.go

@@ -967,6 +967,14 @@ func TestInternalPackageErrorsAreHandled(t *testing.T) {
 	tg.run("list", "./testdata/testinternal3")
 }
 
+func TestInternalCache(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata/testinternal4"))
+	tg.runFail("build", "p")
+	tg.grepStderr("internal", "did not fail to build p")
+}
+
 func TestMoveGit(t *testing.T) {
 	testMove(t, "git", "rsc.io/pdf", "pdf", "rsc.io/pdf/.git/config")
 }

src/cmd/go/pkg.go

@@ -882,7 +882,13 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		deps[path] = p1
 		imports = append(imports, p1)
 		for _, dep := range p1.deps {
-			deps[dep.ImportPath] = dep
+			// Do not overwrite entries installed by direct import
+			// just above this loop. Those have stricter constraints
+			// about internal and vendor visibility and may contain
+			// errors that we need to preserve.
+			if deps[dep.ImportPath] == nil {
+				deps[dep.ImportPath] = dep
+			}
 		}
 		if p1.Incomplete {
 			p.Incomplete = true

src/cmd/go/testdata/testinternal4/src/p/p.go

@@ -0,0 +1,6 @@
+package p
+
+import (
+	_ "q/internal/x"
+	_ "q/j"
+)

src/cmd/go/testdata/testinternal4/src/q/internal/x/x.go

@@ -0,0 +1 @@
+package x

src/cmd/go/testdata/testinternal4/src/q/j/j.go

@@ -0,0 +1,3 @@
+package j
+
+import _ "q/internal/x"