flag: recognize "0s" as the zero value for a flag.Duration

Implemented by using a reflect-based approach to recognize the zero
value of any non-interface type that implements flag.Value.  Interface
types will fall back to the old code.

Fixes #15904.

Change-Id: I594c3bfb30e9ab1aca3e008ef7f70be20aa41a0b
Reviewed-on: https://go-review.googlesource.com/23581
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -68,6 +68,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"reflect"
 	"sort"
 	"strconv"
 	"time"
@@ -378,7 +379,21 @@ func Set(name, value string) error {
 
 // isZeroValue guesses whether the string represents the zero
 // value for a flag. It is not accurate but in practice works OK.
-func isZeroValue(value string) bool {
+func isZeroValue(flag *Flag, value string) bool {
+	// Build a zero value of the flag's Value type, and see if the
+	// result of calling its String method equals the value passed in.
+	// This works unless the Value type is itself an interface type.
+	typ := reflect.TypeOf(flag.Value)
+	var z reflect.Value
+	if typ.Kind() == reflect.Ptr {
+		z = reflect.New(typ.Elem())
+	} else {
+		z = reflect.Zero(typ)
+	}
+	if value == z.Interface().(Value).String() {
+		return true
+	}
+
 	switch value {
 	case "false":
 		return true
@@ -449,7 +464,7 @@ func (f *FlagSet) PrintDefaults() {
 			s += "\n    \t"
 		}
 		s += usage
-		if !isZeroValue(flag.DefValue) {
+		if !isZeroValue(flag, flag.DefValue) {
 			if _, ok := flag.Value.(*stringValue); ok {
 				// put quotes on the value
 				s += fmt.Sprintf(" (default %q)", flag.DefValue)

src/flag/flag_test.go

@@ -393,7 +393,7 @@ const defaultOutput = `  -A	for bootstrapping, allow 'any' type
   -Z int
     	an int that defaults to zero
   -maxT timeout
-    	set timeout for dial (default 0s)
+    	set timeout for dial
 `
 
 func TestPrintDefaults(t *testing.T) {