Support required combined with xor groups.

Fixes #112.

README.md

@@ -446,7 +446,7 @@ Tag                    | Description
 `mapsep:"X"`           | Separator for maps (defaults to ";"). May be `none` to disable splitting.
 `enum:"X,Y,..."`       | Set of valid values allowed for this flag.
 `group:"X"`            | Logical group for a flag or command.
-`xor:"X,Y,..."`        | Exclusive OR groups for flags. Only one flag in the group can be used which is restricted within the same command.
+`xor:"X,Y,..."`        | Exclusive OR groups for flags. Only one flag in the group can be used which is restricted within the same command. When combined with `required`, at least one of the `xor` group will be required.
 `prefix:"X"`           | Prefix for all sub-flags.
 `set:"K=V"`            | Set a variable for expansion by child elements. Multiples can occur.
 `embed:""`             | If present, this field's children will be embedded in the parent. Useful for composition.

context.go

@@ -743,17 +743,32 @@ func (c *Context) PrintUsage(summary bool) error {
 }
 
 func checkMissingFlags(flags []*Flag) error {
+	xorGroup := map[string][]string{}
 	missing := []string{}
 	for _, flag := range flags {
 		if !flag.Required || flag.Set {
 			continue
 		}
-		missing = append(missing, flag.Summary())
+		if len(flag.Xor) > 0 {
+			for _, xor := range flag.Xor {
+				xorGroup[xor] = append(xorGroup[xor], flag.Summary())
+			}
+		} else {
+			missing = append(missing, flag.Summary())
+		}
 	}
+	for _, flags := range xorGroup {
+		if len(flags) > 1 {
+			missing = append(missing, strings.Join(flags, " or "))
+		}
+	}
+
 	if len(missing) == 0 {
 		return nil
 	}
 
+	sort.Strings(missing)
+
 	return fmt.Errorf("missing flags: %s", strings.Join(missing, ", "))
 }
 

kong_test.go

@@ -884,6 +884,26 @@ func TestMultiXor(t *testing.T) {
 	require.EqualError(t, err, "--hello and --two can't be used together")
 }
 
+func TestXorRequired(t *testing.T) {
+	var cli struct {
+		One   bool `xor:"one,two" required:""`
+		Two   bool `xor:"one" required:""`
+		Three bool `xor:"two" required:""`
+		Four  bool `required:""`
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--one"})
+	require.EqualError(t, err, "missing flags: --four")
+
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--two"})
+	require.EqualError(t, err, "missing flags: --four, --one or --three")
+
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{})
+	require.EqualError(t, err, "missing flags: --four, --one or --three, --one or --two")
+}
+
 func TestEnumSequence(t *testing.T) {
 	var cli struct {
 		State []string `enum:"a,b,c" default:"a"`