go/types: don't expose types.Alias kind field - only used by go/types

Change-Id: I8a28a88a655d9929f8641f71573dc01dc53be00f
Reviewed-on: https://go-review.googlesource.com/32443Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/gcimporter/bimport.go

@@ -264,7 +264,7 @@ func (p *importer) obj(tag int) {
 	}
 
 	if aliasName != "" {
-		p.declare(types.NewAlias(aliasPos, p.pkgList[0], aliasName, 0, obj))
+		p.declare(types.NewAlias(aliasPos, p.pkgList[0], aliasName, obj))
 	}
 }
 

src/go/types/object.go

@@ -218,15 +218,16 @@ func (*Func) isDependency()     {} // a function may be a dependency of an initi
 // An Alias represents a declared alias.
 type Alias struct {
 	object
-	kind token.Token // token.CONST, token.TYPE, token.VAR, or token.FUNC
-	orig Object      // aliased constant, type, variable, or function
+	orig Object      // aliased constant, type, variable, or function; never an alias
+	kind token.Token // token.CONST, token.TYPE, token.VAR, or token.FUNC (type-checking internal use only)
 }
 
-func NewAlias(pos token.Pos, pkg *Package, name string, kind token.Token, orig Object) *Alias {
-	return &Alias{object{pos: pos, pkg: pkg, name: name}, kind, orig}
+func NewAlias(pos token.Pos, pkg *Package, name string, orig Object) *Alias {
+	return &Alias{object{pos: pos, pkg: pkg, name: name}, orig, token.ILLEGAL}
 }
 
-func (obj *Alias) Kind() token.Token { return obj.kind }
+// Orig returns the aliased object, or nil if there was an error.
+// The returned object is never an Alias.
 func (obj *Alias) Orig() Object { return obj.orig }
 
 // A Label represents a declared label.

src/go/types/resolver.go

@@ -275,7 +275,8 @@ func (check *Checker) collectObjects() {
 						}
 
 					case *ast.AliasSpec:
-						obj := NewAlias(s.Name.Pos(), pkg, s.Name.Name, d.Tok, nil)
+						obj := NewAlias(s.Name.Pos(), pkg, s.Name.Name, nil)
+						obj.kind = d.Tok
 						check.declarePkgObj(s.Name, obj, &declInfo{file: fileScope, init: s.Orig})
 
 					case *ast.ValueSpec: