cmd/go: diagnose trivial test import cycle

Was detecting only non-trivial ones.

Fixes #9690.

Change-Id: I662d81dd4818ddf29592057c090805772c84287b
Reviewed-on: https://go-review.googlesource.com/12147Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/go_test.go

@@ -1848,6 +1848,9 @@ func TestGoTestDetectsTestOnlyImportCycles(t *testing.T) {
 	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
 	tg.runFail("test", "-c", "testcycle/p3")
 	tg.grepStderr("import cycle not allowed in test", "go test testcycle/p3 produced unexpected error")
+
+	tg.runFail("test", "-c", "testcycle/q1")
+	tg.grepStderr("import cycle not allowed in test", "go test testcycle/q1 produced unexpected error")
 }
 
 func TestGoTestFooTestWorks(t *testing.T) {

src/cmd/go/test.go

@@ -578,7 +578,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		if p1.Error != nil {
 			return nil, nil, nil, p1.Error
 		}
-		if contains(p1.Deps, p.ImportPath) {
+		if contains(p1.Deps, p.ImportPath) || p1.ImportPath == p.ImportPath {
 			// Same error that loadPackage returns (via reusePackage) in pkg.go.
 			// Can't change that code, because that code is only for loading the
 			// non-test copy of a package.

src/cmd/go/testdata/src/testcycle/q1/q1.go

+package q1

src/cmd/go/testdata/src/testcycle/q1/q1_test.go

+package q1
+
+import "testing"
+import _ "testcycle/q1"
+
+func Test(t *testing.T) {}