Allow merging configs with different types of leaf values

viper.go

@@ -1780,17 +1780,6 @@ func mergeMaps(
 
 		svType := reflect.TypeOf(sv)
 		tvType := reflect.TypeOf(tv)
-		if tvType != nil && svType != tvType { // Allow for the target to be nil
-			v.logger.Error(
-				"svType != tvType",
-				"key", sk,
-				"st", svType,
-				"tt", tvType,
-				"sv", sv,
-				"tv", tv,
-			)
-			continue
-		}
 
 		v.logger.Trace(
 			"processing",
@@ -1804,13 +1793,27 @@ func mergeMaps(
 		switch ttv := tv.(type) {
 		case map[interface{}]interface{}:
 			v.logger.Trace("merging maps (must convert)")
-			tsv := sv.(map[interface{}]interface{})
+			tsv, ok := sv.(map[interface{}]interface{})
+			if !ok {
+				v.logger.Error(
+					"Could not cast sv to map[interface{}]interface{}; key=%s, st=%v, tt=%v, sv=%v, tv=%v",
+					sk, svType, tvType, sv, tv)
+				continue
+			}
+
 			ssv := castToMapStringInterface(tsv)
 			stv := castToMapStringInterface(ttv)
 			mergeMaps(ssv, stv, ttv)
 		case map[string]interface{}:
 			v.logger.Trace("merging maps")
-			mergeMaps(sv.(map[string]interface{}), ttv, nil)
+			tsv, ok := sv.(map[string]interface{})
+			if !ok {
+				jww.ERROR.Printf(
+					"Could not cast sv to map[string]interface{}; key=%s, st=%v, tt=%v, sv=%v, tv=%v",
+					sk, svType, tvType, sv, tv)
+				continue
+			}
+			mergeMaps(tsv, ttv, nil)
 		default:
 			v.logger.Trace("setting value")
 			tgt[tk] = sv

viper_test.go

@@ -1912,6 +1912,22 @@ hello:
 fu: bar
 `)
 
+var jsonMergeExampleTgt = []byte(`
+{
+	"hello": {
+		"pop": 123456
+	}
+}
+`)
+
+var jsonMergeExampleSrc = []byte(`
+{
+	"hello": {
+		"pop": "pop str"
+	}
+}
+`)
+
 func TestMergeConfig(t *testing.T) {
 	v := New()
 	v.SetConfigType("yml")
@@ -1984,6 +2000,22 @@ func TestMergeConfig(t *testing.T) {
 	}
 }
 
+func TestMergeConfigOverrideType(t *testing.T) {
+	v := New()
+	v.SetConfigType("json")
+	if err := v.ReadConfig(bytes.NewBuffer(jsonMergeExampleTgt)); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := v.MergeConfig(bytes.NewBuffer(jsonMergeExampleSrc)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetString("hello.pop"); pop != "pop str" {
+		t.Fatalf("pop != \"pop str\", = %s", pop)
+	}
+}
+
 func TestMergeConfigNoMerge(t *testing.T) {
 	v := New()
 	v.SetConfigType("yml")