go/importer: implement importing of exported aliases

Fixes #17592.

Change-Id: I914fa8c0729012990878b6e5c3e99b0f9b0e2be8
Reviewed-on: https://go-review.googlesource.com/32350
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/gcimporter/bimport.go

@@ -222,22 +222,33 @@ func (p *importer) declare(obj types.Object) {
 }
 
 func (p *importer) obj(tag int) {
+	var aliasPos token.Pos
+	var aliasName string
+	if tag == aliasTag {
+		aliasPos = p.pos()
+		aliasName = p.string()
+		tag = p.tagOrIndex()
+	}
+
+	var obj types.Object
 	switch tag {
 	case constTag:
 		pos := p.pos()
 		pkg, name := p.qualifiedName()
 		typ := p.typ(nil)
 		val := p.value()
-		p.declare(types.NewConst(pos, pkg, name, typ, val))
+		obj = types.NewConst(pos, pkg, name, typ, val)
+		p.declare(obj)
 
 	case typeTag:
-		_ = p.typ(nil)
+		obj = p.typ(nil).(*types.Named).Obj()
 
 	case varTag:
 		pos := p.pos()
 		pkg, name := p.qualifiedName()
 		typ := p.typ(nil)
-		p.declare(types.NewVar(pos, pkg, name, typ))
+		obj = types.NewVar(pos, pkg, name, typ)
+		p.declare(obj)
 
 	case funcTag:
 		pos := p.pos()
@@ -245,11 +256,16 @@ func (p *importer) obj(tag int) {
 		params, isddd := p.paramList()
 		result, _ := p.paramList()
 		sig := types.NewSignature(nil, params, result, isddd)
-		p.declare(types.NewFunc(pos, pkg, name, sig))
+		obj = types.NewFunc(pos, pkg, name, sig)
+		p.declare(obj)
 
 	default:
 		errorf("unexpected object tag %d", tag)
 	}
+
+	if aliasName != "" {
+		p.declare(types.NewAlias(aliasPos, p.pkgList[0], aliasName, 0, obj))
+	}
 }
 
 func (p *importer) pos() token.Pos {
@@ -845,7 +861,11 @@ const (
 	fractionTag // not used by gc
 	complexTag
 	stringTag
+	nilTag     // only used by gc (appears in exported inlined function bodies)
 	unknownTag // not used by gc (only appears in packages with errors)
+
+	// Aliases
+	aliasTag
 )
 
 var predeclared = []types.Type{

src/go/internal/gcimporter/testdata/exports.go

@@ -9,6 +9,8 @@ package exports
 
 import (
 	"go/ast"
+	"go/build"
+	"math"
 )
 
 // Issue 3682: Correctly read dotted identifiers from export data.
@@ -27,6 +29,10 @@ const (
 	C7     = `bar\n`
 )
 
+const (
+	C8 => math.Pi
+)
+
 type (
 	T1  int
 	T2  [10]int
@@ -75,9 +81,19 @@ type (
 	T28 func(T28) T28
 )
 
+type (
+	T29 => ast.File
+	T30 => build.Context
+)
+
 var (
 	V0 int
-	V1 = -991.0
+	V1         = -991.0
+	V2 float32 = 1.2
+)
+
+var (
+	V3 => build.Default
 )
 
 func F1()         {}
@@ -87,3 +103,7 @@ func F4() float32 { return 0 }
 func F5(a, b, c int, u, v, w struct{ x, y T1 }, more ...interface{}) (p, q, r chan<- T10)
 
 func (p *T1) M1()
+
+func F6 => math.Sin
+func F7 => ast.IsExported
+func F8 => build.Import