Fix DecodeHook support for decoding map from struct

mapstructure.go

@@ -414,18 +414,18 @@ func NewDecoder(config *DecoderConfig) (*Decoder, error) {
 // Decode decodes the given raw interface to the target pointer specified
 // by the configuration.
 func (d *Decoder) Decode(input interface{}) error {
-	return d.decode("", input, reflect.ValueOf(d.config.Result).Elem())
+	return d.decode("", input, reflect.ValueOf(d.config.Result).Elem(), true)
 }
 
 // Decodes an unknown data type into a specific reflection value.
-func (d *Decoder) decode(name string, input interface{}, outVal reflect.Value) error {
+func (d *Decoder) decode(name string, input interface{}, outVal reflect.Value, skipTypedNil bool) error {
 	var inputVal reflect.Value
 	if input != nil {
 		inputVal = reflect.ValueOf(input)
 
 		// We need to check here if input is a typed nil. Typed nils won't
 		// match the "input == nil" below so we check that here.
-		if inputVal.Kind() == reflect.Ptr && inputVal.IsNil() {
+		if skipTypedNil && inputVal.Kind() == reflect.Ptr && inputVal.IsNil() {
 			input = nil
 		}
 	}
@@ -529,7 +529,7 @@ func (d *Decoder) decodeBasic(name string, data interface{}, val reflect.Value)
 
 		// Decode. If we have an error then return. We also return right
 		// away if we're not a copy because that means we decoded directly.
-		if err := d.decode(name, data, elem); err != nil || !copied {
+		if err := d.decode(name, data, elem, true); err != nil || !copied {
 			return err
 		}
 
@@ -842,7 +842,7 @@ func (d *Decoder) decodeMapFromSlice(name string, dataVal reflect.Value, val ref
 	for i := 0; i < dataVal.Len(); i++ {
 		err := d.decode(
 			name+"["+strconv.Itoa(i)+"]",
-			dataVal.Index(i).Interface(), val)
+			dataVal.Index(i).Interface(), val, true)
 		if err != nil {
 			return err
 		}
@@ -878,15 +878,15 @@ func (d *Decoder) decodeMapFromMap(name string, dataVal reflect.Value, val refle
 
 		// First decode the key into the proper type
 		currentKey := reflect.Indirect(reflect.New(valKeyType))
-		if err := d.decode(fieldName, k.Interface(), currentKey); err != nil {
+		if err := d.decode(fieldName, k.Interface(), currentKey, true); err != nil {
 			errors = appendErrors(errors, err)
 			continue
 		}
 
 		// Next decode the data into the proper type
 		v := dataVal.MapIndex(k).Interface()
 		currentVal := reflect.Indirect(reflect.New(valElemType))
-		if err := d.decode(fieldName, v, currentVal); err != nil {
+		if err := d.decode(fieldName, v, currentVal, true); err != nil {
 			errors = appendErrors(errors, err)
 			continue
 		}
@@ -918,9 +918,6 @@ func (d *Decoder) decodeMapFromStruct(name string, dataVal reflect.Value, val re
 		// Next get the actual value of this field and verify it is assignable
 		// to the map value.
 		v := dataVal.Field(i)
-		if !v.Type().AssignableTo(valMap.Type().Elem()) {
-			return fmt.Errorf("cannot assign type '%s' to map value field of type '%s'", v.Type(), valMap.Type().Elem())
-		}
 
 		tagValue := f.Tag.Get(d.config.TagName)
 		keyName := f.Name
@@ -986,7 +983,7 @@ func (d *Decoder) decodeMapFromStruct(name string, dataVal reflect.Value, val re
 			addrVal := reflect.New(vMap.Type())
 			reflect.Indirect(addrVal).Set(vMap)
 
-			err := d.decode(keyName, x.Interface(), reflect.Indirect(addrVal))
+			err := d.decode(keyName, x.Interface(), reflect.Indirect(addrVal), true)
 			if err != nil {
 				return err
 			}
@@ -1004,7 +1001,13 @@ func (d *Decoder) decodeMapFromStruct(name string, dataVal reflect.Value, val re
 			}
 
 		default:
-			valMap.SetMapIndex(reflect.ValueOf(keyName), v)
+			currentVal := reflect.Indirect(reflect.New(valMap.Type().Elem()))
+			err := d.decode(keyName, v.Interface(), currentVal, false)
+			if err != nil {
+				return err
+			}
+
+			valMap.SetMapIndex(reflect.ValueOf(keyName), currentVal)
 		}
 	}
 
@@ -1049,13 +1052,13 @@ func (d *Decoder) decodePtr(name string, data interface{}, val reflect.Value) (b
 			realVal = reflect.New(valElemType)
 		}
 
-		if err := d.decode(name, data, reflect.Indirect(realVal)); err != nil {
+		if err := d.decode(name, data, reflect.Indirect(realVal), true); err != nil {
 			return false, err
 		}
 
 		val.Set(realVal)
 	} else {
-		if err := d.decode(name, data, reflect.Indirect(val)); err != nil {
+		if err := d.decode(name, data, reflect.Indirect(val), true); err != nil {
 			return false, err
 		}
 	}
@@ -1138,7 +1141,7 @@ func (d *Decoder) decodeSlice(name string, data interface{}, val reflect.Value)
 		currentField := valSlice.Index(i)
 
 		fieldName := name + "[" + strconv.Itoa(i) + "]"
-		if err := d.decode(fieldName, currentData, currentField); err != nil {
+		if err := d.decode(fieldName, currentData, currentField, true); err != nil {
 			errors = appendErrors(errors, err)
 		}
 	}
@@ -1205,7 +1208,7 @@ func (d *Decoder) decodeArray(name string, data interface{}, val reflect.Value)
 		currentField := valArray.Index(i)
 
 		fieldName := name + "[" + strconv.Itoa(i) + "]"
-		if err := d.decode(fieldName, currentData, currentField); err != nil {
+		if err := d.decode(fieldName, currentData, currentField, true); err != nil {
 			errors = appendErrors(errors, err)
 		}
 	}
@@ -1410,7 +1413,7 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 			fieldName = name + "." + fieldName
 		}
 
-		if err := d.decode(fieldName, rawMapVal.Interface(), fieldValue); err != nil {
+		if err := d.decode(fieldName, rawMapVal.Interface(), fieldValue, true); err != nil {
 			errors = appendErrors(errors, err)
 		}
 	}

mapstructure_test.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"reflect"
 	"sort"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -2732,6 +2733,50 @@ func TestDecoder_IgnoreUntaggedFields(t *testing.T) {
 	}
 }
 
+func TestDecodeStructToMap_DecodeHook(t *testing.T) {
+	t.Parallel()
+
+	input := struct {
+		Vstring string
+		Vint    int
+	}{
+		"foo",
+		42,
+	}
+
+	decodeHook := func(f, t reflect.Type, v interface{}) (interface{}, error) {
+		if f.Kind() != reflect.Int {
+			return v, nil
+		}
+		val := strconv.FormatInt(int64(v.(int)), 10)
+		return val, nil
+	}
+
+	var result map[string]interface{}
+	config := &DecoderConfig{
+		DecodeHook: decodeHook,
+		Result:     &result,
+	}
+
+	decoder, err := NewDecoder(config)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	err = decoder.Decode(input)
+	if err != nil {
+		t.Fatalf("got an err: %s", err)
+	}
+
+	expected := map[string]interface{}{
+		"Vstring": "foo",
+		"Vint":    "42",
+	}
+	if !reflect.DeepEqual(result, expected) {
+		t.Errorf("Decode call result should be %#v, got %#v", expected, result)
+	}
+}
+
 func testSliceInput(t *testing.T, input map[string]interface{}, expected *Slice) {
 	var result Slice
 	err := Decode(input, &result)