cmd/go: fix -coverpkg not ignoring special directories

The pattern passed to `-coverpkg` when running `go test` would not
ignore directories usually ignored by the `go` command, i.e. those
beginning with "." or "_" are ignored by the go tool, as are directories
named "testdata".

Fix this by adding an explicit check for these (by following a similar
check in `src/cmd/doc/dirs.go`[1]) allowing us to ignore them.

Two tests are added for this change, one is a regression test attempting
to directly replicate the behaviour described in the issue, the other is
updating another test I saw fail when trialling other solutions to this
issue so I thought it worthwhile to be explicit about the change there.

See linked issue for a reproduction.

Fixes #66038

[1] https://go.googlesource.com/go/+/16e5d24480dca7ddcbdffb78a8ed5de3e5155dec/src/cmd/doc/dirs.go#136

src/cmd/go/go_test.go

@@ -2778,6 +2778,47 @@ func TestCoverpkgTestOnly(t *testing.T) {
 	tg.grepStdout("coverage: 100", "no coverage")
 }
 
+// regression test for github.com/golang/go/issues/66038
+func TestCoverpkgIngoresSpecialDirs(t *testing.T) {
+	skipIfGccgo(t, "gccgo has no cover tool")
+	tooSlow(t, "links and runs a test binary with coverage enabled")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+
+	wd, err := os.Getwd()
+	tg.check(err)
+	tg.makeTempdir()
+	tg.check(os.Chdir(tg.path(".")))
+	defer func() { tg.check(os.Chdir(wd)) }()
+
+	tg.tempFile("a/a.go", `package a
+		import ( _ "b" )
+		func F(i int) int {
+			return i*i
+		}`)
+	tg.tempFile("a/a_test.go", `
+		package a
+		import ( "testing" )
+		func TestF(t *testing.T) { F(2) }
+	`)
+
+	for _, skipDir := range []string{".dir", "_dir", "testdata", "dir/testdata"} {
+		t.Run(skipDir, func(t *testing.T) {
+			tg.tempFile(fmt.Sprintf("%s/src/b/b.go", skipDir), `package b
+				func G(i int) int {
+					return i*i
+				}`)
+			tg.setenv("GOPATH", tg.path(skipDir))
+
+			tg.run("test", "-coverpkg=./...", "./...")
+
+			tg.grepStderrNot("no packages being tested depend on matches", "bad match message")
+			tg.grepStdout("coverage: 100", "no coverage")
+		})
+	}
+}
+
 // Regression test for golang.org/issue/34499: version command should not crash
 // when executed in a deleted directory on Linux.
 func TestExecInDeletedDir(t *testing.T) {

src/cmd/go/internal/load/flag_test.go

@@ -92,6 +92,7 @@ var ppfTests = []ppfTest{
 	ppfDirTest("./...", 3, "/my/test/dir", "/my/test/dir/sub", "/my/test/dir/sub/sub", "/my/test/other", "/my/test/other/sub"),
 	ppfDirTest("../...", 4, "/my/test/dir", "/my/test/other", "/my/test/dir/sub", "/my/test/other/sub", "/my/other/test"),
 	ppfDirTest("../...sub...", 3, "/my/test/dir/sub", "/my/test/othersub", "/my/test/yellowsubmarine", "/my/other/test"),
+	ppfDirTest("./...", 1, "/my/test/dir", "/my/test/dir/.sub", "/test/dir/_sub", "/test/dir/testdata/sub"),
 }
 
 func ppfDirTest(pattern string, nmatch int, dirs ...string) ppfTest {

src/cmd/go/internal/load/search.go

@@ -42,6 +42,19 @@ func MatchPackage(pattern, cwd string) func(*Package) bool {
 			if rel == ".." || strings.HasPrefix(rel, "../") {
 				return false
 			}
+
+			cwdRel, err := filepath.Rel(cwd, p.Dir)
+			if err == nil {
+				if cwdRel != "." && !strings.HasPrefix(cwdRel, "../") {
+					for _, elem := range strings.Split(cwdRel, string(filepath.Separator)) {
+						// Avoid .foo, _foo, and testdata subdirectory trees.
+						if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
+							return false
+						}
+					}
+				}
+			}
+
 			return matchPath(rel)
 		}
 	case pattern == "all":