Add more intricate TypeByDefaultValue logic

In addition to testing the old code, we're now supporting more generic types.  You could set the default value
to a struct type, and with SetTypeByDefualtValue(true), the struct type will be filled in as expected, so long
as the struct type has matching fields in the data.

viper.go

@@ -592,7 +592,6 @@ func (v *Viper) Get(key string) interface{} {
 
 	valType := val
 	if v.typeByDefValue {
-		// TODO(bep) this branch isn't covered by a single test.
 		path := strings.Split(lcaseKey, v.keyDelim)
 		defVal := v.searchMap(v.defaults, path)
 		if defVal != nil {
@@ -615,8 +614,53 @@ func (v *Viper) Get(key string) interface{} {
 		return cast.ToDuration(val)
 	case []string:
 		return cast.ToStringSlice(val)
+	default:
+		return convert(reflect.ValueOf(val), reflect.TypeOf(valType)).Interface()
+	}
+}
+
+func convert(v reflect.Value, typ reflect.Type) reflect.Value {
+	if v.Kind() == reflect.Interface {
+		return convert(v.Elem(), typ)
+	}
+	switch typ.Kind() {
+	case reflect.Ptr:
+		res := reflect.New(typ.Elem())
+		res.Elem().Set(convert(v, typ.Elem()))
+		return res
+	case reflect.Struct:
+		if v.Kind() != reflect.Map {
+			return v
+		}
+		res := reflect.New(typ).Elem()
+		for i := 0; i < typ.NumField(); i++ {
+			field := typ.Field(i)
+			fieldVal := v.MapIndex(reflect.ValueOf(field.Name))
+			if !fieldVal.IsValid() {
+				fieldVal = v.MapIndex(reflect.ValueOf(strings.ToLower(field.Name)))
+				if !fieldVal.IsValid() {
+					continue
+				}
+			}
+			res.Field(i).Set(convert(fieldVal, field.Type))
+		}
+		return res
+	case reflect.Slice, reflect.Array:
+		if v.Kind() != reflect.Slice {
+			return v
+		}
+		res := reflect.Zero(typ)
+		elemType := typ.Elem()
+		for i := 0; i < v.Len(); i++ {
+			res = reflect.Append(res, convert(v.Index(i), elemType))
+		}
+		return res
+	default:
+		if !v.Type().ConvertibleTo(typ) {
+			return v
+		}
+		return v.Convert(typ)
 	}
-	return val
 }
 
 // Sub returns new Viper instance representing a sub tree of this instance.

viper_test.go

@@ -789,6 +789,67 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, (*Viper)(nil), subv)
 }
 
+var yamlTypeExample = []byte(`
+hobbies:
+- skateboarding
+- snowboarding
+languages: go golang python
+job:
+  salary: 100,000 USD
+  medical: full
+clothes:
+- jacket: leather
+  trousers: denim
+  pants:
+    size: large
+- jacket: denim
+  trousers: slacks
+  pants:
+    size: medium
+`)
+
+type Clothing struct {
+	Jacket   string
+	Trousers string
+	Pants    Pants
+}
+
+type Pants struct {
+	Size string
+}
+
+type Job struct {
+	Salary  string
+	Medical string
+}
+
+func TestTypeByDefaultValue(t *testing.T) {
+	v := New()
+	v.SetConfigType("yaml")
+	v.SetDefault("hobbies", []string{})
+	v.SetDefault("languages", []string{})
+	v.SetDefault("job", &Job{})
+	v.SetDefault("clothes", []Clothing{})
+	v.SetTypeByDefaultValue(true)
+	err := v.ReadConfig(bytes.NewBuffer(yamlTypeExample))
+	assert.NoError(t, err)
+
+	assert.Equal(t, []string{"skateboarding", "snowboarding"}, v.Get("hobbies"))
+	assert.Equal(t, []string{"go", "golang", "python"}, v.Get("languages"))
+	assert.Equal(t, []Clothing{
+		{
+			Jacket:   "leather",
+			Trousers: "denim",
+			Pants:    Pants{Size: "large"},
+		},
+		{
+			Jacket:   "denim",
+			Trousers: "slacks",
+			Pants:    Pants{Size: "medium"},
+		},
+	}, v.Get("clothes"))
+}
+
 var yamlMergeExampleTgt = []byte(`
 hello:
     pop: 37890