fix(binding): handle non-JSON string values in form binding for custom struct types

When a struct field implements json.Unmarshaler, form binding passes the
raw form value directly to json.Unmarshal. Since form values are plain
strings (e.g. "2020/09/23 13:20:49"), not valid JSON, the unmarshal
fails. Fix this by retrying with the value wrapped in JSON quotes when
the initial unmarshal fails.

Fixes #2510

binding/form_mapping.go

@@ -373,9 +373,9 @@ func setWithProperType(val string, value reflect.Value, field reflect.StructFiel
 		case multipart.FileHeader:
 			return nil
 		}
-		return json.API.Unmarshal(bytesconv.StringToBytes(val), value.Addr().Interface())
+		return unmarshalFieldAsJSON(val, value)
 	case reflect.Map:
-		return json.API.Unmarshal(bytesconv.StringToBytes(val), value.Addr().Interface())
+		return unmarshalFieldAsJSON(val, value)
 	case reflect.Ptr:
 		if !value.Elem().IsValid() {
 			value.Set(reflect.New(value.Type().Elem()))
@@ -548,3 +548,24 @@ func setFormMap(ptr any, form map[string][]string) error {
 
 	return nil
 }
+
+// unmarshalFieldAsJSON tries to unmarshal a form value as JSON. If the initial
+// unmarshal fails, it retries by wrapping the value in double quotes, treating
+// the form value as a JSON string. This is necessary because form values are
+// plain strings (e.g. "2020/09/23 13:20:49") while json.Unmarshal expects
+// valid JSON input (e.g. quoted strings for string-like values).
+func unmarshalFieldAsJSON(val string, value reflect.Value) error {
+	b := bytesconv.StringToBytes(val)
+	if err := json.API.Unmarshal(b, value.Addr().Interface()); err != nil {
+		// The raw form value is not valid JSON. Wrap it in quotes so that
+		// types implementing json.Unmarshaler that expect a JSON string
+		// (e.g. custom datetime types) can decode it successfully.
+		quoted := `"` + val + `"`
+		if errRetry := json.API.Unmarshal(bytesconv.StringToBytes(quoted), value.Addr().Interface()); errRetry != nil {
+			// Return the original error — the quoted attempt was only a
+			// best-effort fallback.
+			return err
+		}
+	}
+	return nil
+}

binding/form_mapping_test.go

@@ -7,6 +7,7 @@ package binding
 import (
 	"encoding"
 	"encoding/hex"
+	"encoding/json"
 	"errors"
 	"mime/multipart"
 	"reflect"
@@ -484,6 +485,50 @@ func TestMappingMapField(t *testing.T) {
 	assert.Equal(t, map[string]int{"one": 1}, s.M)
 }
 
+// customDateTime is a custom type that implements json.Unmarshaler.
+// It expects a JSON-quoted string in "2006/01/02 15:04:05" format.
+type customDateTime time.Time
+
+const customDateTimeFormat = "2006/01/02 15:04:05"
+
+func (t *customDateTime) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+	parsed, err := time.Parse(customDateTimeFormat, s)
+	if err != nil {
+		return err
+	}
+	*t = customDateTime(parsed)
+	return nil
+}
+
+func TestMappingCustomJSONUnmarshalerStructFromForm(t *testing.T) {
+	var s struct {
+		Time customDateTime `form:"Time"`
+	}
+
+	err := mappingByPtr(&s, formSource{"Time": {"2020/09/23 13:20:49"}}, "form")
+	require.NoError(t, err)
+
+	expected, _ := time.Parse(customDateTimeFormat, "2020/09/23 13:20:49")
+	assert.Equal(t, customDateTime(expected), s.Time)
+}
+
+func TestMappingCustomJSONUnmarshalerStructValidJSON(t *testing.T) {
+	// When the form value is already valid JSON, it should still work.
+	var s struct {
+		J struct {
+			I int
+		}
+	}
+
+	err := mappingByPtr(&s, formSource{"J": {`{"I": 9}`}}, "form")
+	require.NoError(t, err)
+	assert.Equal(t, 9, s.J.I)
+}
+
 func TestMappingIgnoredCircularRef(t *testing.T) {
 	type S struct {
 		S *S `form:"-"`