Allow passing value starting with - to custom option

convert.go

@@ -28,6 +28,15 @@ type Unmarshaler interface {
 	UnmarshalFlag(value string) error
 }
 
+// ValueValidator is the interface implemented by types that can validate a
+// flag argument themselves. The provided value is directly passed from the
+// command line.
+type ValueValidator interface {
+	// IsValidValue returns an error if the provided string value is valid for
+	// the flag.
+	IsValidValue(value string) error
+}
+
 func getBase(options multiTag, base int) (int, error) {
 	sbase := options.Get("base")
 

option.go

@@ -389,6 +389,30 @@ func (option *Option) isUnmarshaler() Unmarshaler {
 	return nil
 }
 
+func (option *Option) isValueValidator() ValueValidator {
+	v := option.value
+
+	for {
+		if !v.CanInterface() {
+			break
+		}
+
+		i := v.Interface()
+
+		if u, ok := i.(ValueValidator); ok {
+			return u
+		}
+
+		if !v.CanAddr() {
+			break
+		}
+
+		v = v.Addr()
+	}
+
+	return nil
+}
+
 func (option *Option) isBool() bool {
 	tp := option.value.Type()
 
@@ -507,3 +531,13 @@ func (option *Option) shortAndLongName() string {
 
 	return ret.String()
 }
+
+func (option *Option) isValidValue(arg string) error {
+	if validator := option.isValueValidator(); validator != nil {
+		return validator.IsValidValue(arg)
+	}
+	if argumentIsOption(arg) && !(option.isSignedNumber() && len(arg) > 1 && arg[0] == '-' && arg[1] >= '0' && arg[1] <= '9') {
+		return fmt.Errorf("expected argument for flag `%s', but got option `%s'", option, arg)
+	}
+	return nil
+}

parser.go

@@ -532,8 +532,8 @@ func (p *Parser) parseOption(s *parseState, name string, option *Option, canarg
 		} else {
 			arg = s.pop()
 
-			if argumentIsOption(arg) && !(option.isSignedNumber() && len(arg) > 1 && arg[0] == '-' && arg[1] >= '0' && arg[1] <= '9') {
-				return newErrorf(ErrExpectedArgument, "expected argument for flag `%s', but got option `%s'", option, arg)
+			if validationErr := option.isValidValue(arg); validationErr != nil {
+				return newErrorf(ErrExpectedArgument, validationErr.Error())
 			} else if p.Options&PassDoubleDash != 0 && arg == "--" {
 				return newErrorf(ErrExpectedArgument, "expected argument for flag `%s', but got double dash `--'", option)
 			}

parser_test.go

@@ -1,6 +1,7 @@
 package flags
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -363,6 +364,22 @@ func TestEnvDefaults(t *testing.T) {
 	}
 }
 
+type CustomFlag struct {
+	Value string
+}
+
+func (c *CustomFlag) UnmarshalFlag(s string) error {
+	c.Value = s
+	return nil
+}
+
+func (c *CustomFlag) IsValidValue(s string) error {
+	if !(s == "-1" || s == "-foo") {
+		return errors.New("invalid flag value")
+	}
+	return nil
+}
+
 func TestOptionAsArgument(t *testing.T) {
 	var tests = []struct {
 		args        []string
@@ -419,30 +436,46 @@ func TestOptionAsArgument(t *testing.T) {
 			rest: []string{"-", "-"},
 		},
 		{
-			// Accept arguments which start with '-' if the next character is a digit, for number options only
+			// Accept arguments which start with '-' if the next character is a digit
 			args: []string{"--int-slice", "-3"},
 		},
 		{
-			// Accept arguments which start with '-' if the next character is a digit, for number options only
+			// Accept arguments which start with '-' if the next character is a digit
 			args: []string{"--int16", "-3"},
 		},
 		{
-			// Accept arguments which start with '-' if the next character is a digit, for number options only
+			// Accept arguments which start with '-' if the next character is a digit
 			args: []string{"--float32", "-3.2"},
 		},
 		{
-			// Accept arguments which start with '-' if the next character is a digit, for number options only
+			// Accept arguments which start with '-' if the next character is a digit
 			args: []string{"--float32ptr", "-3.2"},
 		},
+		{
+			// Accept arguments for values that pass the IsValidValue fuction for value validators
+			args: []string{"--custom-flag", "-foo"},
+		},
+		{
+			// Accept arguments for values that pass the IsValidValue fuction for value validators
+			args: []string{"--custom-flag", "-1"},
+		},
+		{
+			// Rejects arguments for values that fail the IsValidValue fuction for value validators
+			args:        []string{"--custom-flag", "-2"},
+			expectError: true,
+			errType:     ErrExpectedArgument,
+			errMsg:      "invalid flag value",
+		},
 	}
 
 	var opts struct {
-		StringSlice []string `long:"string-slice"`
-		IntSlice    []int    `long:"int-slice"`
-		Int16       int16    `long:"int16"`
-		Float32     float32  `long:"float32"`
-		Float32Ptr  *float32 `long:"float32ptr"`
-		OtherOption bool     `long:"other-option" short:"o"`
+		StringSlice []string   `long:"string-slice"`
+		IntSlice    []int      `long:"int-slice"`
+		Int16       int16      `long:"int16"`
+		Float32     float32    `long:"float32"`
+		Float32Ptr  *float32   `long:"float32ptr"`
+		OtherOption bool       `long:"other-option" short:"o"`
+		Custom      CustomFlag `long:"custom-flag" short:"c"`
 	}
 
 	for _, test := range tests {