From 9283d9a1507f931832e7875228ceea2d3007fc87 Mon Sep 17 00:00:00 2001
From: wuyangfan <yangfan.wu@succaiss.com>
Date: Wed, 27 May 2026 00:16:04 +0800
Subject: [PATCH] fix: stop GetIP erroring when the IP flag has a nil default

When an IP flag defaults to nil, Value.String() round-trips as "<nil>"
and ipConv rejected it. Treat empty and "<nil>" as a nil net.IP, matching
Set("") behavior.

Fixes #478.
Fixes #351.

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 ip.go                  |  3 +++
 ip_nil_default_test.go | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 ip_nil_default_test.go

diff --git a/ip.go b/ip.go
index 06b8bcb5..046f48b3 100644
--- a/ip.go
+++ b/ip.go
@@ -32,6 +32,9 @@ func (i *ipValue) Type() string {
 }
 
 func ipConv(sval string) (interface{}, error) {
+	if sval == "" || sval == "<nil>" {
+		return net.IP(nil), nil
+	}
 	ip := net.ParseIP(sval)
 	if ip != nil {
 		return ip, nil
diff --git a/ip_nil_default_test.go b/ip_nil_default_test.go
new file mode 100644
index 00000000..035f14c6
--- /dev/null
+++ b/ip_nil_default_test.go
@@ -0,0 +1,35 @@
+package pflag
+
+import (
+	"net"
+	"testing"
+)
+
+func TestGetIPNilDefault(t *testing.T) {
+	t.Run("IP", func(t *testing.T) {
+		f := NewFlagSet("test", ContinueOnError)
+		f.IP("ip", nil, "IP address")
+
+		ip, err := f.GetIP("ip")
+		if err != nil {
+			t.Fatalf("GetIP returned error: %v", err)
+		}
+		if ip != nil {
+			t.Fatalf("expected nil IP, got %v", ip)
+		}
+	})
+
+	t.Run("IPVar", func(t *testing.T) {
+		f := NewFlagSet("test", ContinueOnError)
+		var ip net.IP
+		f.IPVar(&ip, "ip", nil, "IP address")
+
+		got, err := f.GetIP("ip")
+		if err != nil {
+			t.Fatalf("GetIP returned error: %v", err)
+		}
+		if got != nil {
+			t.Fatalf("expected nil IP, got %v", got)
+		}
+	})
+}