From a37bbb84b7a9ba0cd347bbe1e3612cc3e7b7585b Mon Sep 17 00:00:00 2001
From: Denis Manente <denis.manente@automatic-server.com>
Date: Tue, 20 Oct 2020 19:24:14 +0200
Subject: [PATCH] provide test case for #167

---
 issue170_test.go | 64 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)
 create mode 100644 issue170_test.go

diff --git a/issue170_test.go b/issue170_test.go
new file mode 100644
index 0000000..718ba8c
--- /dev/null
+++ b/issue170_test.go
@@ -0,0 +1,64 @@
+package mergo_test
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/imdario/mergo"
+)
+
+// This test has been stripped to minimally reproduce the core issue of #170. Consider the playground link for an in-depth analysis.
+
+type Issue170A struct{}
+
+type Issue170InMap struct {
+	As []*Issue170A
+}
+
+type Issue170ValueMap struct {
+	InMap map[string]Issue170InMap
+}
+
+type Issue170Transformer struct{}
+
+func (t Issue170Transformer) Transformer(typ reflect.Type) func(dst, src reflect.Value) error {
+	switch typ {
+	case reflect.TypeOf([]*Issue170A{}):
+		return t.transform
+	}
+
+	return nil
+}
+
+func (t Issue170Transformer) transform(dst, src reflect.Value) error {
+	if !dst.CanSet() {
+		return fmt.Errorf("dst.CanSet() is false, so we're unable to Set() the new value later")
+	}
+
+	dstReal := dst.Interface().([]*Issue170A)
+	dst.Set(reflect.ValueOf(dstReal))
+
+	return nil
+}
+
+var Issue170TestData = []struct {
+	Dst Issue170ValueMap
+	Src Issue170ValueMap
+}{
+	{
+		// Each As must have at least an element
+		Dst: Issue170ValueMap{InMap: map[string]Issue170InMap{"foo": {As: []*Issue170A{{}}}}},
+		Src: Issue170ValueMap{InMap: map[string]Issue170InMap{"foo": {As: []*Issue170A{{}}}}},
+	},
+}
+
+func TestIssue170MapWithValue(t *testing.T) {
+	// And additionally should not panic with every test case
+	for _, data := range Issue170TestData {
+		err := mergo.Merge(&data.Dst, data.Src, mergo.WithTransformers(Issue170Transformer{}))
+		if err != nil {
+			t.Errorf("Error while merging %s", err)
+		}
+	}
+}