Preserve current string values in CopyToGoFlagSet

.github/workflows/ci.yaml

@@ -51,4 +51,4 @@ jobs:
       - name: Lint
         uses: golangci/golangci-lint-action@1e7e51e771db61008b38414a730f564565cf7c20 # v9.2.0
         with:
-          version: v2.13
+          version: v2.14

golangflag.go

@@ -127,7 +127,7 @@ func (f *FlagSet) CopyToGoFlagSet(newSet *goflag.FlagSet) {
 
 		switch value := flag.Value.(type) {
 		case *stringValue:
-			newSet.StringVar((*string)(value), flag.Name, flag.DefValue, usage)
+			newSet.StringVar((*string)(value), flag.Name, *(*string)(value), usage)
 		case *intValue:
 			newSet.IntVar((*int)(value), flag.Name, *(*int)(value), usage)
 		case *int64Value:

golangflag_test.go

@@ -116,7 +116,7 @@ func TestToGoflags(t *testing.T) {
 
 	// Check for unexpected additional flags.
 	gfs.VisitAll(func(gf *goflag.Flag) {
-		pf := gfs.Lookup(gf.Name)
+		pf := pfs.Lookup(gf.Name)
 		if pf == nil {
 			t.Errorf("%s: not found in pflag flag set", gf.Name)
 			return
@@ -133,3 +133,49 @@ func TestToGoflags(t *testing.T) {
 		}
 	}
 }
+
+func TestCopyToGoFlagSetPreservesCurrentStringValue(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		args []string
+		want string
+	}{
+		{name: "default", want: "default"},
+		{name: "parsed", args: []string{"--name=parsed"}, want: "parsed"},
+		{name: "empty", args: []string{"--name="}, want: ""},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			pfs := NewFlagSet("source", ContinueOnError)
+			value := pfs.String("name", "default", "a string flag")
+			if err := pfs.Parse(tc.args); err != nil {
+				t.Fatal(err)
+			}
+			gfs := goflag.NewFlagSet("destination", goflag.ContinueOnError)
+			pfs.CopyToGoFlagSet(gfs)
+			if *value != tc.want {
+				t.Fatalf("copy changed source value: got %q, want %q", *value, tc.want)
+			}
+			if got := gfs.Lookup("name").Value.String(); got != tc.want {
+				t.Errorf("copied value = %q, want %q", got, tc.want)
+			}
+			if got := pfs.Lookup("name").DefValue; got != "default" {
+				t.Errorf("source default = %q, want %q", got, "default")
+			}
+			if got := gfs.Lookup("name").DefValue; got != tc.want {
+				t.Errorf("copied default = %q, want current value %q", got, tc.want)
+			}
+			if err := gfs.Set("name", "from-go"); err != nil {
+				t.Fatal(err)
+			}
+			if *value != "from-go" {
+				t.Errorf("Go flag update not shared with source: got %q", *value)
+			}
+			if err := pfs.Set("name", "from-pflag"); err != nil {
+				t.Fatal(err)
+			}
+			if got := gfs.Lookup("name").Value.String(); got != "from-pflag" {
+				t.Errorf("pflag update not shared with copy: got %q", got)
+			}
+		})
+	}
+}