fix issue setting complex options via flags

If the Option[T].Value implements a Setter interface, call it when
assigning a string option, which is common with command line flag usage.

convert.go

@@ -7,6 +7,16 @@ import (
 
 // dst must be a pointer type
 func convertString(src string, dst interface{}) (err error) {
+	// allow destinations that implement the kingping.Value interface:
+	// https://github.com/alecthomas/kingpin/blob/v2.3.2/values.go#L30-L33
+	type setter interface {
+		Set(string) error
+	}
+	// allow destinations that implement the kingpin.Setter interface:
+	// https://github.com/alecthomas/kingpin/blob/v2.3.2/parsers.go#L12-L14
+	type setValuer interface {
+		SetValue(any) error
+	}
 	switch v := dst.(type) {
 	case *bool:
 		*v, err = strconv.ParseBool(src)
@@ -82,6 +92,10 @@ func convertString(src string, dst interface{}) (err error) {
 		*v = tmp
 	case *any:
 		*v = src
+	case setter:
+		return v.Set(src)
+	case setValuer:
+		return v.SetValue(src)
 	default:
 		err = fmt.Errorf("Cannot convert string %q to type %T", src, dst)
 	}

kingpin_test.go

@@ -8,12 +8,23 @@ import (
 	kingpin "gopkg.in/alecthomas/kingpin.v2"
 )
 
+type Stringish struct {
+	value string
+}
+
+// implement the
+func (s *Stringish) Set(v string) error {
+	s.value = v
+	return nil
+}
+
 func TestCommandLine(t *testing.T) {
 	type CommandLineOptions struct {
-		Str1 StringOption     `yaml:"str1,omitempty"`
-		Int1 IntOption        `yaml:"int1,omitempty"`
-		Map1 MapStringOption  `yaml:"map1,omitempty"`
-		Arr1 ListStringOption `yaml:"arr1,omitempty"`
+		Str1 StringOption          `yaml:"str1,omitempty"`
+		Int1 IntOption             `yaml:"int1,omitempty"`
+		Map1 MapStringOption       `yaml:"map1,omitempty"`
+		Arr1 ListStringOption      `yaml:"arr1,omitempty"`
+		Strs ListOption[Stringish] `yaml:"strs,omitempty"`
 	}
 
 	opts := CommandLineOptions{}
@@ -31,7 +42,8 @@ func TestCommandLine(t *testing.T) {
 	app.Flag("int1", "Int1").SetValue(&opts.Int1)
 	app.Flag("map1", "Map1").SetValue(&opts.Map1)
 	app.Flag("arr1", "Arr1").SetValue(&opts.Arr1)
-	_, err = app.Parse([]string{"--int1", "999", "--map1", "k1=v1", "--map1", "k2=v2", "--arr1", "v1", "--arr1", "v2"})
+	app.Flag("str", "Strs").SetValue(&opts.Strs)
+	_, err = app.Parse([]string{"--int1", "999", "--map1", "k1=v1", "--map1", "k2=v2", "--arr1", "v1", "--arr1", "v2", "--str", "abc", "--str", "def"})
 	require.NoError(t, err)
 
 	arr1 := ListStringOption{}
@@ -58,6 +70,10 @@ func TestCommandLine(t *testing.T) {
 			"k2":   {NewSource("override"), true, "v2"},
 		},
 		Arr1: arr1,
+		Strs: ListOption[Stringish]{
+			{NewSource("override"), true, Stringish{"abc"}},
+			{NewSource("override"), true, Stringish{"def"}},
+		},
 	}
 
 	require.Equal(t, expected, opts)