Fix custom types in structs

fftest/vars.go

@@ -2,6 +2,7 @@ package fftest
 
 import (
 	"errors"
+	"flag"
 	"reflect"
 	"strings"
 	"testing"
@@ -93,3 +94,18 @@ func Compare(t *testing.T, want, have *Vars) {
 		}
 	}
 }
+
+type Custom struct {
+	v string
+}
+
+var _ flag.Value = (*Custom)(nil)
+
+func (c *Custom) Set(s string) error {
+	c.v = s
+	return nil
+}
+
+func (c *Custom) String() string {
+	return c.v
+}

flag_set.go

@@ -713,18 +713,26 @@ func (fs *FlagSet) AddStruct(val any) error {
 
 		// Produce a flag.Value representing the field.
 		{
-			var (
-				fieldValAddr      = fieldVal.Addr()
-				fieldValAddrTyp   = fieldValAddr.Type()
-				fieldValAddrIface = fieldValAddr.Interface()
-				flagValueElemTyp  = reflect.TypeOf((*flag.Value)(nil)).Elem()
-			)
-			if fieldValAddrTyp.Implements(flagValueElemTyp) {
-				// The field implements flag.Value, we can use it directly.
-				cfg.Value = fieldValAddrIface.(flag.Value)
+			flagValueElemTyp := reflect.TypeOf((*flag.Value)(nil)).Elem()
+			if fieldVal.IsValid() && fieldVal.CanAddr() && fieldVal.Addr().Type().Implements(flagValueElemTyp) {
+				cfg.Value = fieldVal.Addr().Interface().(flag.Value)
+				if def != "" {
+					if err := cfg.Value.Set(def); err != nil {
+						return fmt.Errorf("%s: set default: %w", fieldName, err)
+					}
+				}
+			} else if fieldVal.IsValid() && fieldVal.Type().Implements(flagValueElemTyp) && fieldVal.CanInterface() {
+				if fieldVal.IsNil() {
+					return fmt.Errorf("%s: nil value for otherwise valid type (%s)", fieldName, fieldTyp.Type.String())
+				}
+				cfg.Value = fieldVal.Interface().(flag.Value)
+				if def != "" {
+					if err := cfg.Value.Set(def); err != nil {
+						return fmt.Errorf("%s: set default: %w", fieldName, err)
+					}
+				}
 			} else {
-				// Try to construct a new flag value.
-				v, err := ffval.NewValueReflect(fieldValAddrIface, def)
+				v, err := ffval.NewValueReflect(fieldVal.Addr().Interface(), def)
 				if err != nil {
 					return fmt.Errorf("%s: %w", fieldName, err)
 				}

flag_set_test.go

@@ -571,6 +571,59 @@ func TestFlagSet_StructEmbedded(t *testing.T) {
 	}
 }
 
+func TestFlagSet_StructCustom(t *testing.T) {
+	t.Parallel()
+
+	type A struct {
+		Foo fftest.Custom  `ff:"long=foo, default=abc"`
+		Bar *fftest.Custom `ff:"long=bar, default=def"`
+	}
+
+	t.Run("valid no args", func(t *testing.T) {
+		fs := ff.NewFlagSet(t.Name())
+		aflags := A{Bar: &fftest.Custom{}}
+		if err := fs.AddStruct(&aflags); err != nil {
+			t.Fatalf("AddStruct(&aflags): %v", err)
+		}
+		if err := ff.Parse(fs, []string{}); err != nil {
+			t.Fatal(err)
+		}
+		if want, have := "abc", aflags.Foo.String(); want != have {
+			t.Errorf("Foo: want %q, have %q", want, have)
+		}
+		if want, have := "def", aflags.Bar.String(); want != have {
+			t.Errorf("Bar: want %q, have %q", want, have)
+		}
+	})
+
+	t.Run("valid with args", func(t *testing.T) {
+		fs := ff.NewFlagSet(t.Name())
+		aflags := A{Bar: &fftest.Custom{}}
+		if err := fs.AddStruct(&aflags); err != nil {
+			t.Fatalf("AddStruct(&aflags): %v", err)
+		}
+		if err := ff.Parse(fs, []string{"--foo=123", "--bar", "456"}); err != nil {
+			t.Fatal(err)
+		}
+		if want, have := "123", aflags.Foo.String(); want != have {
+			t.Errorf("Foo: want %q, have %q", want, have)
+		}
+		if want, have := "456", aflags.Bar.String(); want != have {
+			t.Errorf("Bar: want %q, have %q", want, have)
+		}
+	})
+
+	t.Run("invalid", func(t *testing.T) {
+		fs := ff.NewFlagSet(t.Name())
+		aflags := A{} // nil Bar
+		if err := fs.AddStruct(&aflags); err == nil {
+			t.Fatalf("AddStruct(&aflags): wanted err, got none")
+		} else {
+			t.Logf("AddStruct(&aflags): got expected error (%v)", err)
+		}
+	})
+}
+
 func TestFlagSet_Std(t *testing.T) {
 	t.Parallel()