package types2
-// This file will eventually define an Alias type.
-// For now it declares asNamed. Once Alias types
-// exist, asNamed will need to indirect through
-// them as needed.
+import "fmt"
+
+// An Alias represents an alias type.
+// Whether or not Alias types are created is controlled by the
+// gotypesalias setting with the GODEBUG environment variable.
+// For gotypesalias=1, alias declarations produce an Alias type.
+// Otherwise, the alias information is only in the type name,
+// which points directly to the actual (aliased) type.
+type Alias struct {
+ obj *TypeName // corresponding declared alias object
+ fromRHS Type // RHS of type alias declaration; may be an alias
+ actual Type // actual (aliased) type; never an alias
+}
+
+// NewAlias creates a new Alias type with the given type name and rhs.
+// rhs must not be nil.
+func NewAlias(obj *TypeName, rhs Type) *Alias {
+ return (*Checker)(nil).newAlias(obj, rhs)
+}
+
+func (a *Alias) Obj() *TypeName { return a.obj }
+func (a *Alias) Underlying() Type { return a.actual.Underlying() }
+func (a *Alias) String() string { return TypeString(a, nil) }
+
+// Type accessors
+
+// Unalias returns t if it is not an alias type;
+// otherwise it follows t's alias chain until it
+// reaches a non-alias type which is then returned.
+// Consequently, the result is never an alias type.
+func Unalias(t Type) Type {
+ if a0, _ := t.(*Alias); a0 != nil {
+ if a0.actual != nil {
+ return a0.actual
+ }
+ for a := a0; ; {
+ t = a.fromRHS
+ a, _ = t.(*Alias)
+ if a == nil {
+ break
+ }
+ }
+ if t == nil {
+ panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
+ }
+ a0.actual = t
+ }
+ return t
+}
// asNamed returns t as *Named if that is t's
// actual type. It returns nil otherwise.
func asNamed(t Type) *Named {
- n, _ := t.(*Named)
+ n, _ := Unalias(t).(*Named)
return n
}
+
+// newAlias creates a new Alias type with the given type name and rhs.
+// rhs must not be nil.
+func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
+ assert(rhs != nil)
+ a := &Alias{obj, rhs, nil}
+ if obj.typ == nil {
+ obj.typ = a
+ }
+
+ // Ensure that a.actual is set at the end of type checking.
+ if check != nil {
+ check.needsCleanup(a)
+ }
+
+ return a
+}
+
+func (a *Alias) cleanup() {
+ Unalias(a)
+}