go/types: more robust imports

- imported objects don't have position information
- gc exported data contains non-exported objects at
  the top-level, guard against them
- better error message when dot-imports conflict
  with local declarations

R=adonovan, r
CC=golang-dev
https://golang.org/cl/7379052

src/pkg/go/types/expr.go

@@ -900,8 +900,11 @@ 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)
-				if exp == nil {
-					check.errorf(e.Sel.Pos(), "cannot refer to unexported %s", 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)
 					goto Error
 				}
 				check.register(e.Sel, exp)

src/pkg/go/types/objects.go

@@ -89,7 +89,13 @@ func (obj *TypeName) GetType() Type { return obj.Type }
 func (obj *Var) GetType() Type      { return obj.Type }
 func (obj *Func) GetType() Type     { return obj.Type }
 
-func (obj *Package) GetPos() token.Pos { return obj.spec.Pos() }
+func (obj *Package) GetPos() token.Pos {
+	if obj.spec != nil {
+		return obj.spec.Pos()
+	}
+	return token.NoPos
+}
+
 func (obj *Const) GetPos() token.Pos {
 	for _, n := range obj.spec.Names {
 		if n.Name == obj.Name {
@@ -98,7 +104,13 @@ func (obj *Const) GetPos() token.Pos {
 	}
 	return token.NoPos
 }
-func (obj *TypeName) GetPos() token.Pos { return obj.spec.Pos() }
+func (obj *TypeName) GetPos() token.Pos {
+	if obj.spec != nil {
+		return obj.spec.Pos()
+	}
+	return token.NoPos
+}
+
 func (obj *Var) GetPos() token.Pos {
 	switch d := obj.decl.(type) {
 	case *ast.Field:
@@ -122,7 +134,12 @@ func (obj *Var) GetPos() token.Pos {
 	}
 	return token.NoPos
 }
-func (obj *Func) GetPos() token.Pos { return obj.decl.Name.Pos() }
+func (obj *Func) GetPos() token.Pos {
+	if obj.decl != nil && obj.decl.Name != nil {
+		return obj.decl.Name.Pos()
+	}
+	return token.NoPos
+}
 
 func (*Package) anObject()  {}
 func (*Const) anObject()    {}

src/pkg/go/types/resolve.go

@@ -11,7 +11,7 @@ import (
 	"strconv"
 )
 
-func (check *checker) declareObj(scope, altScope *Scope, obj Object) {
+func (check *checker) declareObj(scope, altScope *Scope, obj Object, dotImport token.Pos) {
 	alt := scope.Insert(obj)
 	if alt == nil && altScope != nil {
 		// see if there is a conflicting declaration in altScope
@@ -19,8 +19,22 @@ func (check *checker) declareObj(scope, altScope *Scope, obj Object) {
 	}
 	if alt != nil {
 		prevDecl := ""
+
+		// for dot-imports, local declarations are declared first - swap messages
+		if dotImport.IsValid() {
 			if pos := alt.GetPos(); pos.IsValid() {
-			prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", check.fset.Position(pos))
+				check.errorf(pos, fmt.Sprintf("%s redeclared in this block by dot-import at %s",
+					obj.GetName(), check.fset.Position(dotImport)))
+				return
+			}
+
+			// get by w/o other position
+			check.errorf(dotImport, fmt.Sprintf("dot-import redeclares %s", obj.GetName()))
+			return
+		}
+
+		if pos := alt.GetPos(); pos.IsValid() {
+			prevDecl = fmt.Sprintf("\n\tother declaration at %s", check.fset.Position(pos))
 		}
 		check.errorf(obj.GetPos(), fmt.Sprintf("%s redeclared in this block%s", obj.GetName(), prevDecl))
 	}
@@ -137,7 +151,12 @@ func (check *checker) resolve(importer Importer) (methods []*ast.FuncDecl) {
 			if name == "." {
 				// merge imported scope with file scope
 				for _, obj := range imp.Scope.Entries {
-					check.declareObj(fileScope, pkg.Scope, obj)
+					// gcimported package scopes contain non-exported
+					// objects such as types used in partially exported
+					// objects - do not accept them
+					if ast.IsExported(obj.GetName()) {
+						check.declareObj(fileScope, pkg.Scope, obj, spec.Pos())
+					}
 				}
 				// TODO(gri) consider registering the "." identifier
 				// if we have Context.Ident callbacks for say blank
@@ -149,7 +168,7 @@ func (check *checker) resolve(importer Importer) (methods []*ast.FuncDecl) {
 				// a new object instead; the Decl field is different
 				// for different files)
 				obj := &Package{Name: name, Scope: imp.Scope, spec: spec}
-				check.declareObj(fileScope, pkg.Scope, obj)
+				check.declareObj(fileScope, pkg.Scope, obj, token.NoPos)
 			}
 		}
 

src/pkg/go/types/testdata/decls0.src

@@ -11,8 +11,18 @@ import (
 	// we can have multiple blank imports (was bug)
 	_ "math"
 	_ "net/rpc"
+	// reflect defines a type "flag" which shows up in the gc export data
+	"reflect"
+	. "reflect"
 )
 
+// reflect.flag must not be visible in this package
+type flag int
+type _ reflect /* ERROR "cannot refer to unexported" */ .flag
+
+// dot-imported exported objects may conflict with local objects
+type Value /* ERROR "redeclared in this block by dot-import" */ struct{}
+
 const pi = 3.1415
 
 type (

src/pkg/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.xxx /* ERROR "unexported" */
+	t16 math /* ERROR "unexported" */ .xxx
 	t17 math /* ERROR "not a type" */ .Pi
 	t18 float64 = math.Pi * 10.0
 	t19 int = t1 /* ERROR "cannot call" */ ()