1 files changed, 86 insertions, 0 deletions
diff --git a/src/testing/flag_test.go b/src/testing/flag_test.go
new file mode 100644
index 0000000..483ae65
--- /dev/null
+++ b/src/testing/flag_test.go
@@ -0,0 +1,86 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"flag"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+var testFlagArg = flag.String("test_flag_arg", "", "TestFlag: passing -v option")
+
+const flagTestEnv = "GO_WANT_FLAG_HELPER_PROCESS"
+
+func TestFlag(t *testing.T) {
+	if os.Getenv(flagTestEnv) == "1" {
+		testFlagHelper(t)
+		return
+	}
+
+	testenv.MustHaveExec(t)
+
+	for _, flag := range []string{"", "-test.v", "-test.v=test2json"} {
+		flag := flag
+		t.Run(flag, func(t *testing.T) {
+			t.Parallel()
+			exe, err := os.Executable()
+			if err != nil {
+				exe = os.Args[0]
+			}
+			cmd := exec.Command(exe, "-test.run=TestFlag", "-test_flag_arg="+flag)
+			if flag != "" {
+				cmd.Args = append(cmd.Args, flag)
+			}
+			cmd.Env = append(cmd.Environ(), flagTestEnv+"=1")
+			b, err := cmd.CombinedOutput()
+			if len(b) > 0 {
+				t.Logf("%s", b)
+			}
+			if err != nil {
+				t.Error(err)
+			}
+		})
+	}
+}
+
+// testFlagHelper is called by the TestFlagHelper subprocess.
+func testFlagHelper(t *testing.T) {
+	f := flag.Lookup("test.v")
+	if f == nil {
+		t.Fatal(`flag.Lookup("test.v") failed`)
+	}
+
+	bf, ok := f.Value.(interface{ IsBoolFlag() bool })
+	if !ok {
+		t.Errorf("test.v flag (type %T) does not have IsBoolFlag method", f)
+	} else if !bf.IsBoolFlag() {
+		t.Error("test.v IsBoolFlag() returned false")
+	}
+
+	gf, ok := f.Value.(flag.Getter)
+	if !ok {
+		t.Fatalf("test.v flag (type %T) does not have Get method", f)
+	}
+	v := gf.Get()
+
+	var want any
+	switch *testFlagArg {
+	case "":
+		want = false
+	case "-test.v":
+		want = true
+	case "-test.v=test2json":
+		want = "test2json"
+	default:
+		t.Fatalf("unexpected test_flag_arg %q", *testFlagArg)
+	}
+
+	if v != want {
+		t.Errorf("test.v is %v want %v", v, want)
+	}
+}