Support for embedding interfaces.

.golangci.yml

@@ -33,3 +33,5 @@ issues:
     - 'Error return value of .(.*\.Help|.*\.MarkFlagRequired|(os\.)?std(out|err)\..*|.*Close|.*Flush|os\.Remove(All)?|.*printf?|os\.(Un)?Setenv). is not checked'
     - 'exported method (.*\.MarshalJSON|.*\.UnmarshalJSON) should have comment or be unexported'
     - 'composite literal uses unkeyed fields'
+    - 'bad syntax for struct tag key'
+    - 'bad syntax for struct tag pair'

build.go

@@ -42,17 +42,22 @@ type flattenedField struct {
 }
 
 func flattenedFields(v reflect.Value) (out []flattenedField) {
+	v = reflect.Indirect(v)
 	for i := 0; i < v.NumField(); i++ {
 		ft := v.Type().Field(i)
 		fv := v.Field(i)
 		tag := parseTag(fv, ft)
 		if ft.Anonymous {
+			if fv.Kind() == reflect.Interface {
+				fv = fv.Elem()
+			}
 			sub := flattenedFields(fv)
-			// Assign parent group to children, if they're not otherwise set.
 			for _, subf := range sub {
+				// Assign parent if it's not already set.
 				if subf.tag.Group == "" {
 					subf.tag.Group = tag.Group
 				}
+				// Accumulate prefixes.
 				subf.tag.Prefix = tag.Prefix + subf.tag.Prefix
 			}
 			out = append(out, sub...)

kong_test.go

@@ -590,3 +590,26 @@ func TestAnonymousPrefix(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, "moo", cli.Flag)
 }
+
+type TestInterface interface {
+	SomeMethod()
+}
+
+type TestImpl struct {
+	Flag string
+}
+
+func (t *TestImpl) SomeMethod() {}
+
+func TestEmbedInterface(t *testing.T) {
+	type CLI struct {
+		SomeFlag string
+		TestInterface
+	}
+	cli := &CLI{TestInterface: &TestImpl{}}
+	p := mustNew(t, cli)
+	_, err := p.Parse([]string{"--some-flag=foo", "--flag=yes"})
+	require.NoError(t, err)
+	require.Equal(t, "foo", cli.SomeFlag)
+	require.Equal(t, "yes", cli.TestInterface.(*TestImpl).Flag)
+}