Fix: decode nil value in slice decoder (#2102)

common/structure/structure.go

@@ -159,9 +159,19 @@ func (d *Decoder) decodeSlice(name string, data any, val reflect.Value) error {
 		for valSlice.Len() <= i {
 			valSlice = reflect.Append(valSlice, reflect.Zero(valElemType))
 		}
-		currentField := valSlice.Index(i)
-
 		fieldName := fmt.Sprintf("%s[%d]", name, i)
+		if currentData == nil {
+			// in weakly type mode, null will convert to zero value
+			if d.option.WeaklyTypedInput {
+				continue
+			}
+			// in non-weakly type mode, null will convert to nil if element's zero value is nil, otherwise return an error
+			if elemKind := valElemType.Kind(); elemKind == reflect.Map || elemKind == reflect.Slice {
+				continue
+			}
+			return fmt.Errorf("'%s' can not be null", fieldName)
+		}
+		currentField := valSlice.Index(i)
 		if err := d.decode(fieldName, currentData, currentField); err != nil {
 			return err
 		}

common/structure/structure_test.go

@@ -137,3 +137,45 @@ func TestStructure_Nest(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Equal(t, s.BazOptional, goal)
 }
+
+func TestStructure_SliceNilValue(t *testing.T) {
+	rawMap := map[string]any{
+		"foo": 1,
+		"bar": []any{"bar", nil},
+	}
+
+	goal := &BazSlice{
+		Foo: 1,
+		Bar: []string{"bar", ""},
+	}
+
+	s := &BazSlice{}
+	err := weakTypeDecoder.Decode(rawMap, s)
+	assert.Nil(t, err)
+	assert.Equal(t, goal.Bar, s.Bar)
+
+	s = &BazSlice{}
+	err = decoder.Decode(rawMap, s)
+	assert.NotNil(t, err)
+}
+
+func TestStructure_SliceNilValueComplex(t *testing.T) {
+	rawMap := map[string]any{
+		"bar": []any{map[string]any{"bar": "foo"}, nil},
+	}
+
+	s := &struct {
+		Bar []map[string]any `test:"bar"`
+	}{}
+
+	err := decoder.Decode(rawMap, s)
+	assert.Nil(t, err)
+	assert.Nil(t, s.Bar[1])
+
+	ss := &struct {
+		Bar []Baz `test:"bar"`
+	}{}
+
+	err = decoder.Decode(rawMap, ss)
+	assert.NotNil(t, err)
+}