validator_instance.go

package validator

import (
	"context"
	"errors"
	"fmt"
	"reflect"
	"strings"
	"sync"
	"time"

	ut "github.com/go-playground/universal-translator"
)

const (
	defaultTagName        = "validate"
	utf8HexComma          = "0x2C"
	utf8Pipe              = "0x7C"
	tagSeparator          = ","
	orSeparator           = "|"
	tagKeySeparator       = "="
	structOnlyTag         = "structonly"
	noStructLevelTag      = "nostructlevel"
	omitzero              = "omitzero"
	omitempty             = "omitempty"
	omitnil               = "omitnil"
	isdefault             = "isdefault"
	requiredWithoutAllTag = "required_without_all"
	requiredWithoutTag    = "required_without"
	requiredWithTag       = "required_with"
	requiredWithAllTag    = "required_with_all"
	requiredIfTag         = "required_if"
	requiredUnlessTag     = "required_unless"
	skipUnlessTag         = "skip_unless"
	excludedWithoutAllTag = "excluded_without_all"
	excludedWithoutTag    = "excluded_without"
	excludedWithTag       = "excluded_with"
	excludedWithAllTag    = "excluded_with_all"
	excludedIfTag         = "excluded_if"
	excludedUnlessTag     = "excluded_unless"
	skipValidationTag     = "-"
	diveTag               = "dive"
	keysTag               = "keys"
	endKeysTag            = "endkeys"
	requiredTag           = "required"
	namespaceSeparator    = "."
	leftBracket           = "["
	rightBracket          = "]"
	restrictedTagChars    = ".[],|=+()`~!@#$%^&*\\\"/?<>{}"
	restrictedAliasErr    = "Alias '%s' either contains restricted characters or is the same as a restricted tag needed for normal operation"
	restrictedTagErr      = "Tag '%s' either contains restricted characters or is the same as a restricted tag needed for normal operation"
)

var (
	timeDurationType = reflect.TypeOf(time.Duration(0))
	timeType         = reflect.TypeOf(time.Time{})

	byteSliceType = reflect.TypeOf([]byte{})

	defaultCField = &cField{namesEqual: true}
)

// FilterFunc is the type used to filter fields using
// StructFiltered(...) function.
// returning true results in the field being filtered/skipped from
// validation
type FilterFunc func(ns []byte) bool

// CustomTypeFunc allows for overriding or adding custom field type handler functions
// field = field value of the type to return a value to be validated
// example Valuer from sql drive see https://golang.org/src/database/sql/driver/types.go?s=1210:1293#L29
type CustomTypeFunc func(field reflect.Value) interface{}

// TagNameFunc allows for adding of a custom tag name parser
type TagNameFunc func(field reflect.StructField) string

type internalValidationFuncWrapper struct {
	fn                 FuncCtx
	runValidationOnNil bool
}

// Validate contains the validator settings and cache
type Validate struct {
	tagName                string
	pool                   *sync.Pool
	tagNameFunc            TagNameFunc
	structLevelFuncs       map[reflect.Type]StructLevelFuncCtx
	customFuncs            map[reflect.Type]CustomTypeFunc
	aliases                map[string]string
	validations            map[string]internalValidationFuncWrapper
	transTagFunc           map[ut.Translator]map[string]TranslationFunc // map[<locale>]map[<tag>]TranslationFunc
	rules                  map[reflect.Type]map[string]string
	tagCache               *tagCache
	structCache            *structCache
	hasCustomFuncs         bool
	hasTagNameFunc         bool
	requiredStructEnabled  bool
	privateFieldValidation bool
}

// New returns a new instance of 'validate' with sane defaults.
// Validate is designed to be thread-safe and used as a singleton instance.
// It caches information about your struct and validations,
// in essence only parsing your validation tags once per struct type.
// Using multiple instances neglects the benefit of caching.
func New(options ...Option) *Validate {
	tc := new(tagCache)
	tc.m.Store(make(map[string]*cTag))

	sc := new(structCache)
	sc.m.Store(make(map[reflect.Type]*cStruct))

	v := &Validate{
		tagName:     defaultTagName,
		aliases:     make(map[string]string, len(bakedInAliases)),
		validations: make(map[string]internalValidationFuncWrapper, len(bakedInValidators)),
		tagCache:    tc,
		structCache: sc,
	}

	// must copy alias validators for separate validations to be used in each validator instance
	for k, val := range bakedInAliases {
		v.RegisterAlias(k, val)
	}

	// must copy validators for separate validations to be used in each instance
	for k, val := range bakedInValidators {
		switch k {
		// these require that even if the value is nil that the validation should run, omitempty still overrides this behaviour
		case requiredIfTag, requiredUnlessTag, requiredWithTag, requiredWithAllTag, requiredWithoutTag, requiredWithoutAllTag,
			excludedIfTag, excludedUnlessTag, excludedWithTag, excludedWithAllTag, excludedWithoutTag, excludedWithoutAllTag,
			skipUnlessTag:
			_ = v.registerValidation(k, wrapFunc(val), true, true)
		default:
			// no need to error check here, baked in will always be valid
			_ = v.registerValidation(k, wrapFunc(val), true, false)
		}
	}

	v.pool = &sync.Pool{
		New: func() interface{} {
			return &validate{
				v:        v,
				ns:       make([]byte, 0, 64),
				actualNs: make([]byte, 0, 64),
				misc:     make([]byte, 32),
			}
		},
	}

	for _, o := range options {
		o(v)
	}
	return v
}

// SetTagName allows for changing of the default tag name of 'validate'
func (v *Validate) SetTagName(name string) {
	v.tagName = name
}

// ValidateMapCtx validates a map using a map of validation rules and allows passing of contextual
// validation information via context.Context.
func (v Validate) ValidateMapCtx(ctx context.Context, data map[string]interface{}, rules map[string]interface{}) map[string]interface{} {
	errs := make(map[string]interface{})
	for field, rule := range rules {
		if ruleObj, ok := rule.(map[string]interface{}); ok {
			if dataObj, ok := data[field].(map[string]interface{}); ok {
				err := v.ValidateMapCtx(ctx, dataObj, ruleObj)
				if len(err) > 0 {
					errs[field] = err
				}
			} else if dataObjs, ok := data[field].([]map[string]interface{}); ok {
				for _, obj := range dataObjs {
					err := v.ValidateMapCtx(ctx, obj, ruleObj)
					if len(err) > 0 {
						errs[field] = err
					}
				}
			} else {
				errs[field] = errors.New("The field: '" + field + "' is not a map to dive")
			}
		} else if ruleStr, ok := rule.(string); ok {
			err := v.VarCtx(ctx, data[field], ruleStr)
			if err != nil {
				errs[field] = err
			}
		}
	}
	return errs
}

// ValidateMap validates map data from a map of tags
func (v *Validate) ValidateMap(data map[string]interface{}, rules map[string]interface{}) map[string]interface{} {
	return v.ValidateMapCtx(context.Background(), data, rules)
}

// RegisterTagNameFunc registers a function to get alternate names for StructFields.
//
// eg. to use the names which have been specified for JSON representations of structs, rather than normal Go field names:
//
//	validate.RegisterTagNameFunc(func(fld reflect.StructField) string {
//	    name := strings.SplitN(fld.Tag.Get("json"), ",", 2)[0]
//	    // skip if tag key says it should be ignored
//	    if name == "-" {
//	        return ""
//	    }
//	    return name
//	})
func (v *Validate) RegisterTagNameFunc(fn TagNameFunc) {
	v.tagNameFunc = fn
	v.hasTagNameFunc = true
}

// RegisterValidation adds a validation with the given tag
//
// NOTES:
// - if the key already exists, the previous validation function will be replaced.
// - this method is not thread-safe it is intended that these all be registered prior to any validation
func (v *Validate) RegisterValidation(tag string, fn Func, callValidationEvenIfNull ...bool) error {
	return v.RegisterValidationCtx(tag, wrapFunc(fn), callValidationEvenIfNull...)
}

// RegisterValidationCtx does the same as RegisterValidation on accepts a FuncCtx validation
// allowing context.Context validation support.
func (v *Validate) RegisterValidationCtx(tag string, fn FuncCtx, callValidationEvenIfNull ...bool) error {
	var nilCheckable bool
	if len(callValidationEvenIfNull) > 0 {
		nilCheckable = callValidationEvenIfNull[0]
	}
	return v.registerValidation(tag, fn, false, nilCheckable)
}

// RegisterAlias registers a mapping of a single validation tag that
// defines a common or complex set of validation(s) to simplify adding validation
// to structs.
//
// NOTE: this function is not thread-safe it is intended that these all be registered prior to any validation
func (v *Validate) RegisterAlias(alias, tags string) {
	_, ok := restrictedTags[alias]

	if ok || strings.ContainsAny(alias, restrictedTagChars) {
		panic(fmt.Sprintf(restrictedAliasErr, alias))
	}

	v.aliases[alias] = tags
}

// RegisterStructValidation registers a StructLevelFunc against a number of types.
//
// NOTE:
// - this method is not thread-safe it is intended that these all be registered prior to any validation
func (v *Validate) RegisterStructValidation(fn StructLevelFunc, types ...interface{}) {
	v.RegisterStructValidationCtx(wrapStructLevelFunc(fn), types...)
}

// RegisterStructValidationCtx registers a StructLevelFuncCtx against a number of types and allows passing
// of contextual validation information via context.Context.
//
// NOTE:
// - this method is not thread-safe it is intended that these all be registered prior to any validation
func (v *Validate) RegisterStructValidationCtx(fn StructLevelFuncCtx, types ...interface{}) {
	if v.structLevelFuncs == nil {
		v.structLevelFuncs = make(map[reflect.Type]StructLevelFuncCtx)
	}

	for _, t := range types {
		tv := reflect.ValueOf(t)
		if tv.Kind() == reflect.Ptr {
			t = reflect.Indirect(tv).Interface()
		}

		v.structLevelFuncs[reflect.TypeOf(t)] = fn
	}
}

// RegisterStructValidationMapRules registers validate map rules.
// Be aware that map validation rules supersede those defined on a/the struct if present.
//
// NOTE: this method is not thread-safe it is intended that these all be registered prior to any validation
func (v *Validate) RegisterStructValidationMapRules(rules map[string]string, types ...interface{}) {
	if v.rules == nil {
		v.rules = make(map[reflect.Type]map[string]string)
	}

	deepCopyRules := make(map[string]string)
	for i, rule := range rules {
		deepCopyRules[i] = rule
	}

	for _, t := range types {
		typ := reflect.TypeOf(t)

		if typ.Kind() == reflect.Ptr {
			typ = typ.Elem()
		}

		if typ.Kind() != reflect.Struct {
			continue
		}
		v.rules[typ] = deepCopyRules
	}
}

// RegisterCustomTypeFunc registers a CustomTypeFunc against a number of types
//
// NOTE: this method is not thread-safe it is intended that these all be registered prior to any validation
func (v *Validate) RegisterCustomTypeFunc(fn CustomTypeFunc, types ...interface{}) {
	if v.customFuncs == nil {
		v.customFuncs = make(map[reflect.Type]CustomTypeFunc)
	}

	for _, t := range types {
		v.customFuncs[reflect.TypeOf(t)] = fn
	}

	v.hasCustomFuncs = true
}

// RegisterTranslation registers translations against the provided tag.
func (v *Validate) RegisterTranslation(tag string, trans ut.Translator, registerFn RegisterTranslationsFunc, translationFn TranslationFunc) (err error) {
	if v.transTagFunc == nil {
		v.transTagFunc = make(map[ut.Translator]map[string]TranslationFunc)
	}

	if err = registerFn(trans); err != nil {
		return
	}

	m, ok := v.transTagFunc[trans]
	if !ok {
		m = make(map[string]TranslationFunc)
		v.transTagFunc[trans] = m
	}

	m[tag] = translationFn

	return
}

// Struct validates a structs exposed fields, and automatically validates nested structs, unless otherwise specified.
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) Struct(s interface{}) error {
	return v.StructCtx(context.Background(), s)
}

// StructCtx validates a structs exposed fields, and automatically validates nested structs, unless otherwise specified
// and also allows passing of context.Context for contextual validation information.
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructCtx(ctx context.Context, s interface{}) (err error) {
	val := reflect.ValueOf(s)
	top := val

	if val.Kind() == reflect.Ptr && !val.IsNil() {
		val = val.Elem()
	}

	if val.Kind() != reflect.Struct || val.Type().ConvertibleTo(timeType) {
		return &InvalidValidationError{Type: reflect.TypeOf(s)}
	}

	// good to validate
	vd := v.pool.Get().(*validate)
	vd.top = top
	vd.isPartial = false
	// vd.hasExcludes = false // only need to reset in StructPartial and StructExcept

	vd.validateStruct(ctx, top, val, val.Type(), vd.ns[0:0], vd.actualNs[0:0], nil)

	if len(vd.errs) > 0 {
		err = vd.errs
		vd.errs = nil
	}

	v.pool.Put(vd)

	return
}

// StructFiltered validates a structs exposed fields, that pass the FilterFunc check and automatically validates
// nested structs, unless otherwise specified.
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructFiltered(s interface{}, fn FilterFunc) error {
	return v.StructFilteredCtx(context.Background(), s, fn)
}

// StructFilteredCtx validates a structs exposed fields, that pass the FilterFunc check and automatically validates
// nested structs, unless otherwise specified and also allows passing of contextual validation information via
// context.Context
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructFilteredCtx(ctx context.Context, s interface{}, fn FilterFunc) (err error) {
	val := reflect.ValueOf(s)
	top := val

	if val.Kind() == reflect.Ptr && !val.IsNil() {
		val = val.Elem()
	}

	if val.Kind() != reflect.Struct || val.Type().ConvertibleTo(timeType) {
		return &InvalidValidationError{Type: reflect.TypeOf(s)}
	}

	// good to validate
	vd := v.pool.Get().(*validate)
	vd.top = top
	vd.isPartial = true
	vd.ffn = fn
	// vd.hasExcludes = false // only need to reset in StructPartial and StructExcept

	vd.validateStruct(ctx, top, val, val.Type(), vd.ns[0:0], vd.actualNs[0:0], nil)

	if len(vd.errs) > 0 {
		err = vd.errs
		vd.errs = nil
	}

	v.pool.Put(vd)

	return
}

// StructPartial validates the fields passed in only, ignoring all others.
// Fields may be provided in a namespaced fashion relative to the  struct provided
// eg. NestedStruct.Field or NestedArrayField[0].Struct.Name
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructPartial(s interface{}, fields ...string) error {
	return v.StructPartialCtx(context.Background(), s, fields...)
}

// StructPartialCtx validates the fields passed in only, ignoring all others and allows passing of contextual
// validation information via context.Context
// Fields may be provided in a namespaced fashion relative to the  struct provided
// eg. NestedStruct.Field or NestedArrayField[0].Struct.Name
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructPartialCtx(ctx context.Context, s interface{}, fields ...string) (err error) {
	val := reflect.ValueOf(s)
	top := val

	if val.Kind() == reflect.Ptr && !val.IsNil() {
		val = val.Elem()
	}

	if val.Kind() != reflect.Struct || val.Type().ConvertibleTo(timeType) {
		return &InvalidValidationError{Type: reflect.TypeOf(s)}
	}

	// good to validate
	vd := v.pool.Get().(*validate)
	vd.top = top
	vd.isPartial = true
	vd.ffn = nil
	vd.hasExcludes = false
	vd.includeExclude = make(map[string]struct{})

	typ := val.Type()
	name := typ.Name()

	for _, k := range fields {
		flds := strings.Split(k, namespaceSeparator)
		if len(flds) > 0 {
			vd.misc = append(vd.misc[0:0], name...)
			// Don't append empty name for unnamed structs
			if len(vd.misc) != 0 {
				vd.misc = append(vd.misc, '.')
			}

			for _, s := range flds {
				idx := strings.Index(s, leftBracket)

				if idx != -1 {
					for idx != -1 {
						vd.misc = append(vd.misc, s[:idx]...)
						vd.includeExclude[string(vd.misc)] = struct{}{}

						idx2 := strings.Index(s, rightBracket)
						idx2++
						vd.misc = append(vd.misc, s[idx:idx2]...)
						vd.includeExclude[string(vd.misc)] = struct{}{}
						s = s[idx2:]
						idx = strings.Index(s, leftBracket)
					}
				} else {
					vd.misc = append(vd.misc, s...)
					vd.includeExclude[string(vd.misc)] = struct{}{}
				}

				vd.misc = append(vd.misc, '.')
			}
		}
	}

	vd.validateStruct(ctx, top, val, typ, vd.ns[0:0], vd.actualNs[0:0], nil)

	if len(vd.errs) > 0 {
		err = vd.errs
		vd.errs = nil
	}

	v.pool.Put(vd)

	return
}

// StructExcept validates all fields except the ones passed in.
// Fields may be provided in a namespaced fashion relative to the  struct provided
// i.e. NestedStruct.Field or NestedArrayField[0].Struct.Name
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructExcept(s interface{}, fields ...string) error {
	return v.StructExceptCtx(context.Background(), s, fields...)
}

// StructExceptCtx validates all fields except the ones passed in and allows passing of contextual
// validation information via context.Context
// Fields may be provided in a namespaced fashion relative to the  struct provided
// i.e. NestedStruct.Field or NestedArrayField[0].Struct.Name
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
func (v *Validate) StructExceptCtx(ctx context.Context, s interface{}, fields ...string) (err error) {
	val := reflect.ValueOf(s)
	top := val

	if val.Kind() == reflect.Ptr && !val.IsNil() {
		val = val.Elem()
	}

	if val.Kind() != reflect.Struct || val.Type().ConvertibleTo(timeType) {
		return &InvalidValidationError{Type: reflect.TypeOf(s)}
	}

	// good to validate
	vd := v.pool.Get().(*validate)
	vd.top = top
	vd.isPartial = true
	vd.ffn = nil
	vd.hasExcludes = true
	vd.includeExclude = make(map[string]struct{})

	typ := val.Type()
	name := typ.Name()

	for _, key := range fields {
		vd.misc = vd.misc[0:0]

		if len(name) > 0 {
			vd.misc = append(vd.misc, name...)
			vd.misc = append(vd.misc, '.')
		}

		vd.misc = append(vd.misc, key...)
		vd.includeExclude[string(vd.misc)] = struct{}{}
	}

	vd.validateStruct(ctx, top, val, typ, vd.ns[0:0], vd.actualNs[0:0], nil)

	if len(vd.errs) > 0 {
		err = vd.errs
		vd.errs = nil
	}

	v.pool.Put(vd)

	return
}

// Var validates a single variable using tag style validation.
// eg.
// var i int
// validate.Var(i, "gt=1,lt=10")
//
// WARNING: a struct can be passed for validation eg. time.Time is a struct or
// if you have a custom type and have registered a custom type handler, so must
// allow it; however unforeseen validations will occur if trying to validate a
// struct that is meant to be passed to 'validate.Struct'
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
// validate Array, Slice and maps fields which may contain more than one error
func (v *Validate) Var(field interface{}, tag string) error {
	return v.VarCtx(context.Background(), field, tag)
}

// VarCtx validates a single variable using tag style validation and allows passing of contextual
// validation information via context.Context.
// eg.
// var i int
// validate.Var(i, "gt=1,lt=10")
//
// WARNING: a struct can be passed for validation eg. time.Time is a struct or
// if you have a custom type and have registered a custom type handler, so must
// allow it; however unforeseen validations will occur if trying to validate a
// struct that is meant to be passed to 'validate.Struct'
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
// validate Array, Slice and maps fields which may contain more than one error
func (v *Validate) VarCtx(ctx context.Context, field interface{}, tag string) (err error) {
	if len(tag) == 0 || tag == skipValidationTag {
		return nil
	}

	ctag := v.fetchCacheTag(tag)

	val := reflect.ValueOf(field)
	vd := v.pool.Get().(*validate)
	vd.top = val
	vd.isPartial = false
	vd.traverseField(ctx, val, val, vd.ns[0:0], vd.actualNs[0:0], defaultCField, ctag)

	if len(vd.errs) > 0 {
		err = vd.errs
		vd.errs = nil
	}
	v.pool.Put(vd)
	return
}

// VarWithValue validates a single variable, against another variable/field's value using tag style validation
// eg.
// s1 := "abcd"
// s2 := "abcd"
// validate.VarWithValue(s1, s2, "eqcsfield") // returns true
//
// WARNING: a struct can be passed for validation eg. time.Time is a struct or
// if you have a custom type and have registered a custom type handler, so must
// allow it; however unforeseen validations will occur if trying to validate a
// struct that is meant to be passed to 'validate.Struct'
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
// validate Array, Slice and maps fields which may contain more than one error
func (v *Validate) VarWithValue(field interface{}, other interface{}, tag string) error {
	return v.VarWithValueCtx(context.Background(), field, other, tag)
}

// VarWithValueCtx validates a single variable, against another variable/field's value using tag style validation and
// allows passing of contextual validation information via context.Context.
// eg.
// s1 := "abcd"
// s2 := "abcd"
// validate.VarWithValue(s1, s2, "eqcsfield") // returns true
//
// WARNING: a struct can be passed for validation eg. time.Time is a struct or
// if you have a custom type and have registered a custom type handler, so must
// allow it; however unforeseen validations will occur if trying to validate a
// struct that is meant to be passed to 'validate.Struct'
//
// It returns InvalidValidationError for bad values passed in and nil or ValidationErrors as error otherwise.
// You will need to assert the error if it's not nil eg. err.(validator.ValidationErrors) to access the array of errors.
// validate Array, Slice and maps fields which may contain more than one error
func (v *Validate) VarWithValueCtx(ctx context.Context, field interface{}, other interface{}, tag string) (err error) {
	if len(tag) == 0 || tag == skipValidationTag {
		return nil
	}
	ctag := v.fetchCacheTag(tag)
	otherVal := reflect.ValueOf(other)
	vd := v.pool.Get().(*validate)
	vd.top = otherVal
	vd.isPartial = false
	vd.traverseField(ctx, otherVal, reflect.ValueOf(field), vd.ns[0:0], vd.actualNs[0:0], defaultCField, ctag)

	if len(vd.errs) > 0 {
		err = vd.errs
		vd.errs = nil
	}
	v.pool.Put(vd)
	return
}

func (v *Validate) registerValidation(tag string, fn FuncCtx, bakedIn bool, nilCheckable bool) error {
	if len(tag) == 0 {
		return errors.New("function Key cannot be empty")
	}

	if fn == nil {
		return errors.New("function cannot be empty")
	}

	_, ok := restrictedTags[tag]
	if !bakedIn && (ok || strings.ContainsAny(tag, restrictedTagChars)) {
		panic(fmt.Sprintf(restrictedTagErr, tag))
	}
	v.validations[tag] = internalValidationFuncWrapper{fn: fn, runValidationOnNil: nilCheckable}
	return nil
}