From df0700a1e899196d8020767981b82623c8738932 Mon Sep 17 00:00:00 2001
From: Mike Lundy <mike@fluffypenguin.org>
Date: Tue, 8 May 2018 14:59:37 -0700
Subject: [PATCH] Look through interfaces and pointers when determining
 emptiness

When dealing with map[string]interface{}, the common case for loading
arbitrary data from yaml/json, empty strings are not overwritten because
they appear as a non-empty interface containing an empty string. This
change causes non-nil interfaces to be examined for emptiness.
---
 mergo.go     |  7 ++++++-
 pr80_test.go | 18 ++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 pr80_test.go

diff --git a/mergo.go b/mergo.go
index 785618c..a82fea2 100644
--- a/mergo.go
+++ b/mergo.go
@@ -45,7 +45,12 @@ func isEmptyValue(v reflect.Value) bool {
 		return v.Uint() == 0
 	case reflect.Float32, reflect.Float64:
 		return v.Float() == 0
-	case reflect.Interface, reflect.Ptr, reflect.Func:
+	case reflect.Interface, reflect.Ptr:
+		if v.IsNil() {
+			return true
+		}
+		return isEmptyValue(v.Elem())
+	case reflect.Func:
 		return v.IsNil()
 	case reflect.Invalid:
 		return true
diff --git a/pr80_test.go b/pr80_test.go
new file mode 100644
index 0000000..0b3220f
--- /dev/null
+++ b/pr80_test.go
@@ -0,0 +1,18 @@
+package mergo
+
+import (
+	"testing"
+)
+
+type mapInterface map[string]interface{}
+
+func TestMergeMapsEmptyString(t *testing.T) {
+	a := mapInterface{"s": ""}
+	b := mapInterface{"s": "foo"}
+	if err := Merge(&a, b); err != nil {
+		t.Fatal(err)
+	}
+	if a["s"] != "foo" {
+		t.Fatalf("b not merged in properly: a.s.Value(%s) != expected(%s)", a["s"], "foo")
+	}
+}