Fix xor + required combo.

See #112.

context.go

@@ -743,14 +743,23 @@ func (c *Context) PrintUsage(summary bool) error {
 }
 
 func checkMissingFlags(flags []*Flag) error {
+	xorGroupSet := map[string]bool{}
 	xorGroup := map[string][]string{}
 	missing := []string{}
 	for _, flag := range flags {
+		if flag.Set {
+			for _, xor := range flag.Xor {
+				xorGroupSet[xor] = true
+			}
+		}
 		if !flag.Required || flag.Set {
 			continue
 		}
 		if len(flag.Xor) > 0 {
 			for _, xor := range flag.Xor {
+				if xorGroupSet[xor] {
+					continue
+				}
 				xorGroup[xor] = append(xorGroup[xor], flag.Summary())
 			}
 		} else {

kong_test.go

@@ -904,6 +904,21 @@ func TestXorRequired(t *testing.T) {
 	require.EqualError(t, err, "missing flags: --four, --one or --three, --one or --two")
 }
 
+func TestXorRequiredMany(t *testing.T) {
+	var cli struct {
+		One   bool `xor:"one" required:""`
+		Two   bool `xor:"one" required:""`
+		Three bool `xor:"one" required:""`
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--one"})
+	require.NoError(t, err)
+
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{})
+	require.EqualError(t, err, "missing flags: --one or --two or --three")
+}
+
 func TestEnumSequence(t *testing.T) {
 	var cli struct {
 		State []string `enum:"a,b,c" default:"a"`