fix: several issues while copy with custom converter

As mentioned in issue#170 there're several issues in using custom converter:

1. from struct to struct
2. from map to map
3. from slice to slice
4. from embedded slice to embedded slice

All these cases are tested in `copier_issue170_test.go`. There is
a FIXME leaved behind becaused I'm not very familier with types in golang.
It'll be great if someone could fix it for me.

Resolves: #170

copier.go

@@ -215,6 +215,13 @@ func copier(toValue interface{}, fromValue interface{}, opt Option) (err error)
 		return
 	}
 
+	if len(converters) > 0 {
+		if ok, e := set(to, from, opt.DeepCopy, converters); e == nil && ok {
+			// converter supported
+			return
+		}
+	}
+
 	if from.Kind() == reflect.Slice || to.Kind() == reflect.Slice {
 		isSlice = true
 		if from.Kind() == reflect.Slice {
@@ -239,6 +246,27 @@ func copier(toValue interface{}, fromValue interface{}, opt Option) (err error)
 			dest = indirect(to)
 		}
 
+		if len(converters) > 0 {
+			if ok, e := set(dest, source, opt.DeepCopy, converters); e == nil && ok {
+				if isSlice {
+					// FIXME: maybe should check the other types?
+					if to.Type().Elem().Kind() == reflect.Ptr {
+						to.Index(i).Set(dest.Addr())
+					} else {
+						if to.Len() < i+1 {
+							reflect.Append(to, dest)
+						} else {
+							to.Index(i).Set(dest)
+						}
+					}
+				} else {
+					to.Set(dest)
+				}
+
+				continue
+			}
+		}
+
 		destKind := dest.Kind()
 		initDest := false
 		if destKind == reflect.Interface {

copier_issue170_test.go

@@ -0,0 +1,112 @@
+package copier_test
+
+import (
+	"github.com/jinzhu/copier"
+	"reflect"
+	"testing"
+)
+
+type A struct {
+	A int
+}
+type B struct {
+	A int
+	b int
+}
+
+var copied = B{A: 2387483274, b: 128387134}
+
+func newOptWithConverter() copier.Option {
+	return copier.Option{
+		Converters: []copier.TypeConverter{
+			{
+				SrcType: A{},
+				DstType: B{},
+				Fn: func(from interface{}) (interface{}, error) {
+					return copied, nil
+				},
+			},
+		},
+	}
+}
+
+func Test_Struct_With_Converter(t *testing.T) {
+	aa := A{A: 11}
+	bb := B{A: 10, b: 100}
+	err := copier.CopyWithOption(&bb, &aa, newOptWithConverter())
+	if err != nil || !reflect.DeepEqual(copied, bb) {
+		t.Fatalf("Got %v, wanted %v", bb, copied)
+	}
+}
+
+func Test_Map_With_Converter(t *testing.T) {
+	aa := map[string]*A{
+		"a": &A{A: 10},
+	}
+
+	bb := map[string]*B{
+		"a": &B{A: 10, b: 100},
+	}
+
+	err := copier.CopyWithOption(&bb, &aa, newOptWithConverter())
+	if err != nil {
+		t.Fatalf("copy with converter failed: %v", err)
+	}
+
+	for _, v := range bb {
+		wanted := &copied
+		if !reflect.DeepEqual(v, wanted) {
+			t.Fatalf("Got %v, wanted %v", v, wanted)
+		}
+	}
+}
+
+func Test_Slice_With_Converter(t *testing.T) {
+	aa := []*A{
+		&A{A: 10},
+	}
+
+	bb := []*B{
+		&B{A: 10, b: 100},
+	}
+
+	err := copier.CopyWithOption(&bb, &aa, newOptWithConverter())
+
+	if err != nil {
+		t.Fatalf("copy slice error: %v", err)
+	}
+
+	wanted := copied
+	for _, v := range bb {
+		temp := v
+		if !reflect.DeepEqual(*temp, wanted) {
+			t.Fatalf("Got %v, wanted %v", *temp, wanted)
+		}
+	}
+}
+
+func Test_Slice_Embedded_With_Converter(t *testing.T) {
+	aa := struct {
+		A []*A
+	}{
+		A: []*A{&A{A: 10}},
+	}
+
+	bb := struct {
+		A []*B
+	}{
+		A: []*B{&B{A: 10, b: 100}},
+	}
+
+	err := copier.CopyWithOption(&bb, &aa, newOptWithConverter())
+
+	wanted := struct {
+		A []*B
+	}{
+		A: []*B{&copied},
+	}
+
+	if err != nil || !reflect.DeepEqual(bb, wanted) {
+		t.Fatalf("Got %v, wanted %v", bb, wanted)
+	}
+}