go.exp/go/types: better error message for non-existing qualified idents

Fixes golang/go#4967.

R=golang-dev, r, remyoudompheng
CC=golang-dev
https://golang.org/cl/8092048

go/types/expr.go

@@ -1145,11 +1145,14 @@ func (check *checker) rawExpr(x *operand, e ast.Expr, hint Type, iota int, cycle
 		if ident, ok := e.X.(*ast.Ident); ok {
 			if pkg, ok := check.lookup(ident).(*Package); ok {
 				exp := pkg.Scope.Lookup(sel)
-				// gcimported package scopes contain non-exported
-				// objects such as types used in partially exported
-				// objects - do not accept them
-				if exp == nil || !ast.IsExported(exp.GetName()) {
-					check.errorf(e.Pos(), "cannot refer to unexported %s", e)
+				if exp == nil {
+					check.errorf(e.Pos(), "%s not declared by package %s", sel, ident)
+					goto Error
+				} else if !ast.IsExported(exp.GetName()) {
+					// gcimported package scopes contain non-exported
+					// objects such as types used in partially exported
+					// objects - do not accept them
+					check.errorf(e.Pos(), "%s not exported by package %s", sel, ident)
 					goto Error
 				}
 				check.register(e.Sel, exp)

go/types/testdata/decls0.src

@@ -18,7 +18,7 @@ import (
 
 // reflect.flag must not be visible in this package
 type flag int
-type _ reflect /* ERROR "cannot refer to unexported" */ .flag
+type _ reflect /* ERROR "not exported" */ .flag
 
 // dot-imported exported objects may conflict with local objects
 type Value /* ERROR "redeclared in this block by dot-import" */ struct{}

go/types/testdata/decls1.src

@@ -64,7 +64,7 @@ var (
 	t13 int = a /* ERROR "shifted operand" */ << d
 	t14 int = i << j /* ERROR "must be unsigned" */ 
 	t15 math /* ERROR "not in selector" */
-	t16 math /* ERROR "unexported" */ .xxx
+	t16 math /* ERROR "not declared" */ .xxx
 	t17 math /* ERROR "not a type" */ .Pi
 	t18 float64 = math.Pi * 10.0
 	t19 int = t1 /* ERROR "cannot call" */ ()