Don't validate enum values for unused required paths.

Fixes #40.

context.go

@@ -132,10 +132,10 @@ func (c *Context) Empty() bool {
 }
 
 // Validate the current context.
-func (c *Context) Validate() error {
+func (c *Context) Validate() error { // nolint: gocyclo
 	err := Visit(c.Model, func(node Visitable, next Next) error {
 		if value, ok := node.(*Value); ok {
-			if value.Enum != "" {
+			if value.Enum != "" && (!value.Required || value.Default != "") {
 				if err := checkEnum(value, value.Target); err != nil {
 					return err
 				}
@@ -152,6 +152,19 @@ func (c *Context) Validate() error {
 		}
 	}
 	for _, path := range c.Path {
+		var value *Value
+		switch {
+		case path.Flag != nil:
+			value = path.Flag.Value
+
+		case path.Positional != nil:
+			value = path.Positional
+		}
+		if value != nil && value.Tag.Enum != "" {
+			if err := checkEnum(value, value.Target); err != nil {
+				return err
+			}
+		}
 		if err := checkMissingFlags(path.Flags); err != nil {
 			return err
 		}

kong_test.go

@@ -791,3 +791,27 @@ func TestEnumSequence(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []string{"a"}, cli.State)
 }
+
+func TestIssue40EnumAcrossCommands(t *testing.T) {
+	var cli struct {
+		One struct {
+			OneArg string `arg:"" required:""`
+		} `cmd:""`
+		Two struct {
+			TwoArg string `arg:"" enum:"a,b,c" required:""`
+		} `cmd:""`
+		Three struct {
+			ThreeArg string `arg:"" optional:"" default:"a" enum:"a,b,c"`
+		} `cmd:""`
+	}
+
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"one", "two"})
+	require.NoError(t, err)
+	_, err = p.Parse([]string{"two", "d"})
+	require.Error(t, err)
+	_, err = p.Parse([]string{"three", "d"})
+	require.Error(t, err)
+	_, err = p.Parse([]string{"three", "c"})
+	require.NoError(t, err)
+}