lang: types: Add nil type for placeholder

The nil type is being added, but only slightly. It is not meant for real
use in the language. It will not work in all situations, and it isn't
implemented for many methods.

It is being added only for being a "dummy" placeholder value in the
engine when we have an unused value being propagated along an edge.

lang/types/type.go

@@ -51,6 +51,7 @@ const (
 
 // Basic types defined here as a convenience for use with Type.Cmp(X).
 var (
+	TypeNil     = NewType("nil")
 	TypeBool    = NewType("bool")
 	TypeStr     = NewType("str")
 	TypeInt     = NewType("int")
@@ -384,6 +385,10 @@ func NewType(s string) *Type {
 // return a type with correctly unified unification variables.
 func newType(s string, table map[uint]*Elem) *Type {
 	switch s {
+	case "nil":
+		return &Type{
+			Kind: KindNil,
+		}
 	case "bool":
 		return &Type{
 			Kind: KindBool,
@@ -683,6 +688,8 @@ func (obj *Type) New() Value {
 		panic("malformed type")
 	}
 	switch obj.Kind {
+	case KindNil:
+		return NewNil()
 	case KindBool:
 		return NewBool()
 	case KindStr:
@@ -717,6 +724,8 @@ func (obj *Type) String() string {
 // helper function that is used by the real String function.
 func (obj *Type) string(table map[*Elem]uint) string {
 	switch obj.Kind {
+	case KindNil:
+		return "nil"
 	case KindBool:
 		return "bool"
 	case KindStr:
@@ -840,6 +849,8 @@ func (obj *Type) cmp(typ *Type, table1, table2 map[*Elem]uint) error {
 		return fmt.Errorf("base kind does not match (%+v != %+v)", obj.Kind, typ.Kind)
 	}
 	switch obj.Kind {
+	case KindNil:
+		return nil
 	case KindBool:
 		return nil
 	case KindStr:
@@ -1124,6 +1135,8 @@ func (obj *Type) HasVariant() bool {
 		return false
 	}
 	switch obj.Kind {
+	case KindNil:
+		return false
 	case KindBool:
 		return false
 	case KindStr:
@@ -1212,6 +1225,8 @@ func (obj *Type) HasUni() bool {
 	}
 
 	switch obj.Kind {
+	case KindNil:
+		return false
 	case KindBool:
 		return false
 	case KindStr:
@@ -1330,6 +1345,8 @@ func (obj *Type) ComplexCmp(typ *Type) (string, error) {
 
 	// only container types are left to match...
 	switch obj.Kind {
+	case KindNil:
+		return "", nil // regular cmp
 	case KindBool:
 		return "", nil // regular cmp
 	case KindStr: