Expose errors returned by validateFn

errors.go

@@ -2,6 +2,7 @@ package validator
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"reflect"
 	"strings"
@@ -50,6 +51,20 @@ func (ve ValidationErrors) Error() string {
 	return strings.TrimSpace(buff.String())
 }
 
+// Unwrap returns the nested errors reported by individual field validations.
+func (ve ValidationErrors) Unwrap() []error {
+	errs := make([]error, 0, len(ve))
+	for _, fe := range ve {
+		if err := errors.Unwrap(fe); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	if len(errs) == 0 {
+		return nil
+	}
+	return errs
+}
+
 // Translate translates all of the ValidationErrors
 func (ve ValidationErrors) Translate(ut ut.Translator) ValidationErrorsTranslations {
 	trans := make(ValidationErrorsTranslations)
@@ -174,6 +189,7 @@ type fieldError struct {
 	param          string
 	kind           reflect.Kind
 	typ            reflect.Type
+	err            error
 }
 
 // Tag returns the validation tag that failed.
@@ -248,6 +264,11 @@ func (fe *fieldError) Error() string {
 	return fmt.Sprintf(fieldErrMsg, fe.ns, fe.Field(), fe.tag)
 }
 
+// Unwrap returns the underlying error reported by the validation, if any.
+func (fe *fieldError) Unwrap() error {
+	return fe.err
+}
+
 // Translate returns the FieldError's translated error
 // from the provided 'ut.Translator' and registered 'TranslationFunc'
 //

no_validate_fn.go

@@ -16,6 +16,9 @@ func isValidateFn(fl FieldLevel) bool {
 
 	ok, err := tryCallValidateFn(field, validateFn)
 	if err != nil {
+		if v, ok := fl.(*validate); ok {
+			v.fieldErr = err
+		}
 		return false
 	}
 
@@ -48,7 +51,11 @@ func tryCallValidateFn(field reflect.Value, validateFn string) (bool, error) {
 		errorType := reflect.TypeOf((*error)(nil)).Elem()
 
 		if firstReturnValue.Type().Implements(errorType) {
-			return firstReturnValue.IsNil(), nil
+			if firstReturnValue.IsNil() {
+				return true, nil
+			}
+
+			return false, firstReturnValue.Interface().(error)
 		}
 
 		return false, fmt.Errorf("unable to use result of method %q on type %q: %w (got interface %v expect error)",

validator.go

@@ -17,6 +17,7 @@ type validate struct {
 	errs           ValidationErrors
 	includeExclude map[string]struct{} // reset only if StructPartial or StructExcept are called, no need otherwise
 	ffn            FilterFunc
+	fieldErr       error
 	slflParent     reflect.Value // StructLevel & FieldLevel
 	slCurrent      reflect.Value // StructLevel & FieldLevel
 	flField        reflect.Value // StructLevel & FieldLevel
@@ -462,6 +463,7 @@ OUTER:
 		default:
 
 			// set Field Level fields
+			v.fieldErr = nil
 			v.slflParent = parent
 			v.flField = current
 			v.cf = cf
@@ -489,6 +491,7 @@ OUTER:
 						param:          ct.param,
 						kind:           kind,
 						typ:            typ,
+						err:            v.fieldErr,
 					},
 				)
 

validator_test.go

@@ -15644,6 +15644,14 @@ func (r NotRed) DoNothing() {}
 
 func (r NotRed) String() string { return "not red instance" }
 
+var errValidateFnFailed = errors.New("validate function failed")
+
+type ValidateFnFailed struct{}
+
+func (ValidateFnFailed) Validate() error {
+	return errValidateFnFailed
+}
+
 func TestValidateFn(t *testing.T) {
 	t.Run("using pointer", func(t *testing.T) {
 		validate := New()
@@ -15733,6 +15741,28 @@ func TestValidateFn(t *testing.T) {
 		Equal(t, fe.Tag(), "validateFn")
 	})
 
+	t.Run("returned error can be unwrapped", func(t *testing.T) {
+		validate := New()
+
+		type Test struct {
+			Inner ValidateFnFailed `validate:"validateFn"`
+		}
+
+		err := validate.Struct(&Test{})
+		NotEqual(t, err, nil)
+		Equal(t, errors.Is(err, errValidateFnFailed), true)
+
+		errs := err.(ValidationErrors)
+		Equal(t, len(errs), 1)
+
+		fe := errs[0]
+		Equal(t, fe.Field(), "Inner")
+		Equal(t, fe.Namespace(), "Test.Inner")
+		Equal(t, fe.Tag(), "validateFn")
+		Equal(t, errors.Is(fe, errValidateFnFailed), true)
+		Equal(t, errors.Unwrap(fe), errValidateFnFailed)
+	})
+
 	t.Run("try validate method with wrong signature or not existent", func(t *testing.T) {
 		validate := New()