Allow passing value starting with - to custom option

Signed-off-by: Difan Zhao <dzhao@pivotal.io>

parser.go

@@ -515,7 +515,7 @@ 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') {
+			if argumentIsOption(arg) && option.isUnmarshaler() == nil && !(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)
 			} else if p.Options&PassDoubleDash != 0 && arg == "--" {
 				return newErrorf(ErrExpectedArgument, "expected argument for flag `%s', but got double dash `--'", option)

parser_test.go

@@ -343,6 +343,15 @@ func TestEnvDefaults(t *testing.T) {
 	}
 }
 
+type CustomFlag struct {
+	Value string
+}
+
+func (c *CustomFlag) UnmarshalFlag(s string) error {
+	c.Value = s
+	return nil
+}
+
 func TestOptionAsArgument(t *testing.T) {
 	var tests = []struct {
 		args        []string
@@ -399,30 +408,39 @@ 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 which start with '-' if the next character is not a digit, for custom flags
+			args: []string{"--custom-flag", "-foo"},
+		},
+		{
+			// Accept arguments which start with '-' if the next character is a digit, for custom flags
+			args: []string{"--custom-flag", "-1"},
+		},
 	}
 
 	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 {