flag: test IsBoolFlag when creating the usage message

Although I can't think of any reason to do this, it is possible for
a user-defined flag to implement IsBoolFlag but return "false".
This is nuts because checking the interface is satisfied should
obviously be sufficient, but the documentation kinda implies it's
not. And if you try this, you'll discover that the usage message
ignores the return value even though the rest of the package plays
nice. Bother.

So we fix it, as the fix is trivial: call the method when creating
the usage message.

Fixes #53473

Change-Id: I1ac80a876ad5626eebfc5ef6cb972cd3007afaad
Reviewed-on: https://go-review.googlesource.com/c/go/+/431102
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/flag/flag.go

@@ -550,9 +550,11 @@ func UnquoteUsage(flag *Flag) (name string, usage string) {
 	}
 	// No explicit name, so use type if we can find one.
 	name = "value"
-	switch flag.Value.(type) {
+	switch fv := flag.Value.(type) {
 	case boolFlag:
+		if fv.IsBoolFlag() {
 			name = ""
+		}
 	case *durationValue:
 		name = "duration"
 	case *float64Value:

src/flag/flag_test.go

@@ -5,6 +5,7 @@
 package flag_test
 
 import (
+	"bytes"
 	. "flag"
 	"fmt"
 	"internal/testenv"
@@ -355,6 +356,31 @@ func TestUserDefinedBool(t *testing.T) {
 	}
 }
 
+func TestUserDefinedBoolUsage(t *testing.T) {
+	var flags FlagSet
+	flags.Init("test", ContinueOnError)
+	var buf bytes.Buffer
+	flags.SetOutput(&buf)
+	var b boolFlagVar
+	flags.Var(&b, "b", "X")
+	b.count = 0
+	// b.IsBoolFlag() will return true and usage will look boolean.
+	flags.PrintDefaults()
+	got := buf.String()
+	want := "  -b\tX\n"
+	if got != want {
+		t.Errorf("false: want %q; got %q", want, got)
+	}
+	b.count = 4
+	// b.IsBoolFlag() will return false and usage will look non-boolean.
+	flags.PrintDefaults()
+	got = buf.String()
+	want = "  -b\tX\n  -b value\n    \tX\n"
+	if got != want {
+		t.Errorf("false: want %q; got %q", want, got)
+	}
+}
+
 func TestSetOutput(t *testing.T) {
 	var flags FlagSet
 	var buf strings.Builder