testing: fix invalid error message about argument of TestMain

Also, this commit adds a test for ensuring that TestMain(t *testing.T) is a normal test.

Fixes #22388

Change-Id: Iffcb1db5cdcf34b9c822fcdb58f8926535415177
Reviewed-on: https://go-review.googlesource.com/72591
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -2901,6 +2901,21 @@ func TestGoTestFooTestWorks(t *testing.T) {
 	tg.run("test", "testdata/standalone_test.go")
 }
 
+// Issue 22388
+func TestGoTestMainWithWrongSignature(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.runFail("test", "testdata/standalone_main_wrong_test.go")
+	tg.grepStderr(`wrong signature for TestMain, must be: func TestMain\(m \*testing.M\)`, "detected wrong error message")
+}
+
+func TestGoTestMainAsNormalTest(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.run("test", "testdata/standalone_main_normal_test.go")
+	tg.grepBoth(okPattern, "go test did not say ok")
+}
+
 func TestGoTestFlagsAfterPackage(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()

src/cmd/go/internal/test/test.go

@@ -1694,7 +1694,16 @@ func (t *testFuncs) load(filename, pkg string, doImport, seen *bool) error {
 		}
 		name := n.Name.String()
 		switch {
-		case name == "TestMain" && isTestFunc(n, "M"):
+		case name == "TestMain":
+			if isTestFunc(n, "T") {
+				t.Tests = append(t.Tests, testFunc{pkg, name, "", false})
+				*doImport, *seen = true, true
+				continue
+			}
+			err := checkTestFunc(n, "M")
+			if err != nil {
+				return err
+			}
 			if t.TestMain != nil {
 				return errors.New("multiple definitions of TestMain")
 			}

src/cmd/go/testdata/standalone_main_normal_test.go

@@ -0,0 +1,10 @@
+// Copyright 2017 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 standalone_main_normal_test
+
+import "testing"
+
+func TestMain(t *testing.T) {
+}

src/cmd/go/testdata/standalone_main_wrong_test.go

@@ -0,0 +1,10 @@
+// Copyright 2017 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 standalone_main_wrong_test
+
+import "testing"
+
+func TestMain(m *testing.Main) {
+}