cmd/go: show examples with empty output in go test -list

Fixes #21205

Change-Id: I81b001eb42cbf2a5d5b7b82eb63548b22f501be5
Reviewed-on: https://go-review.googlesource.com/52110
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -4314,3 +4314,20 @@ func TestTestRegexps(t *testing.T) {
 		t.Errorf("reduced output:<<<\n%s>>> want:<<<\n%s>>>", have, want)
 	}
 }
+
+func TestListTests(t *testing.T) {
+	var tg *testgoData
+	testWith := func(listName, expected string) func(*testing.T) {
+		return func(t *testing.T) {
+			tg = testgo(t)
+			defer tg.cleanup()
+			tg.run("test", "./testdata/src/testlist/...", fmt.Sprintf("-list=%s", listName))
+			tg.grepStdout(expected, fmt.Sprintf("-test.list=%s returned %q, expected %s", listName, tg.getStdout(), expected))
+		}
+	}
+
+	t.Run("Test", testWith("Test", "TestSimple"))
+	t.Run("Bench", testWith("Benchmark", "BenchmarkSimple"))
+	t.Run("Example1", testWith("Example", "ExampleSimple"))
+	t.Run("Example2", testWith("Example", "ExampleWithEmptyOutput"))
+}

src/cmd/go/testdata/src/testlist/bench_test.go

@@ -0,0 +1,14 @@
+package testlist
+
+import (
+	"fmt"
+	"testing"
+)
+
+func BenchmarkSimplefunc(b *testing.B) {
+	b.StopTimer()
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		_ = fmt.Sprint("Test for bench")
+	}
+}

src/cmd/go/testdata/src/testlist/example_test.go

@@ -0,0 +1,21 @@
+package testlist
+
+import (
+	"fmt"
+)
+
+func ExampleSimple() {
+	fmt.Println("Test with Output.")
+
+	// Output: Test with Output.
+}
+
+func ExampleWithEmptyOutput() {
+	fmt.Println("")
+
+	// Output:
+}
+
+func ExampleNoOutput() {
+	_ = fmt.Sprint("Test with no output")
+}

src/cmd/go/testdata/src/testlist/test_test.go

@@ -0,0 +1,10 @@
+package testlist
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestSimple(t *testing.T) {
+	_ = fmt.Sprint("Test simple")
+}

src/testing/testing.go

@@ -970,7 +970,7 @@ func listTests(matchString func(pat, str string) (bool, error), tests []Internal
 		}
 	}
 	for _, example := range examples {
-		if ok, _ := matchString(*matchList, example.Name); ok && example.Output != "" {
+		if ok, _ := matchString(*matchList, example.Name); ok {
 			fmt.Println(example.Name)
 		}
 	}