add test for keeping zero values with transformer

pr211_test.go

@@ -0,0 +1,36 @@
+package mergo_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/imdario/mergo"
+)
+
+func TestMergeWithTransformerZeroValue(t *testing.T) {
+	// This test specifically tests that a transformer can be used to
+	// prevent overwriting a zero value (in this case a bool). This would fail prior to #211
+	type fooWithBoolPtr struct {
+		b *bool
+	}
+	var Bool = func(b bool) *bool { return &b }
+	a := fooWithBoolPtr{b: Bool(false)}
+	b := fooWithBoolPtr{b: Bool(true)}
+
+	if err := mergo.Merge(&a, &b, mergo.WithTransformers(&transformer{
+		m: map[reflect.Type]func(dst, src reflect.Value) error{
+			reflect.TypeOf(Bool(false)): func(dst, src reflect.Value) error {
+				if dst.CanSet() && dst.IsNil() {
+					dst.Set(src)
+				}
+				return nil
+			},
+		},
+	})); err != nil {
+		t.Error(err)
+	}
+
+	if *a.b != false {
+		t.Errorf("b not merged in properly: a.b(%v) != expected(%v)", a.b, false)
+	}
+}