fix: treat "-vn=" in a short option group as an empty value

Author: kevin9327

command_parse.go

@@ -248,7 +248,9 @@ func (cmd *Command) parseFlags(args Args) (Args, error) {
 					return &stringSliceArgs{posArgs}, err
 				}
 			} else if index == lastIndex { // last flag can take an arg
-				if flagVal == "" {
+				// as with a single flag, an explicit "=" with nothing after
+				// it is an empty value, not a request to read the next arg
+				if flagVal == "" && !valFromEqual {
 					if len(rargs) == 1 {
 						return &stringSliceArgs{posArgs}, fmt.Errorf("%s%s", argumentNotProvidedErrMsg, string(c))
 					}

command_test.go

@@ -6948,6 +6948,48 @@ func TestFlagEqualsEmptyValue(t *testing.T) {
 		assert.Equal(t, "", val)
 		assert.Equal(t, []string{"positional"}, args)
 	})
+
+	t.Run("-vn= in a short option group sets empty string", func(t *testing.T) {
+		var val string
+		var verbose bool
+		var args []string
+
+		cmd := &Command{
+			UseShortOptionHandling: true,
+			Flags: []Flag{
+				&BoolFlag{Name: "v", Destination: &verbose},
+				&StringFlag{Name: "n", Destination: &val, Value: "default"},
+			},
+			Action: func(_ context.Context, cmd *Command) error {
+				args = cmd.Args().Slice()
+				return nil
+			},
+		}
+
+		err := cmd.Run(buildTestContext(t), []string{"app", "-vn=", "positional"})
+		assert.NoError(t, err)
+		assert.True(t, verbose)
+		assert.Equal(t, "", val)
+		assert.Equal(t, []string{"positional"}, args)
+	})
+
+	t.Run("-vn= at the end of a short option group sets empty string", func(t *testing.T) {
+		var val string
+		var verbose bool
+
+		cmd := &Command{
+			UseShortOptionHandling: true,
+			Flags: []Flag{
+				&BoolFlag{Name: "v", Destination: &verbose},
+				&StringFlag{Name: "n", Destination: &val, Value: "default"},
+			},
+		}
+
+		err := cmd.Run(buildTestContext(t), []string{"app", "-vn="})
+		assert.NoError(t, err)
+		assert.True(t, verbose)
+		assert.Equal(t, "", val)
+	})
 }
 
 // TestCommand_NoDefaultCmdArgMatchingFlag tests the argument set