go.tools/go/types: package name must not be blank

Fixes golang/go#8077.

LGTM=adonovan
R=golang-codereviews, adonovan
CC=golang-codereviews
https://golang.org/cl/91640043

go/types/check.go

@@ -162,7 +162,11 @@ func (check *checker) initFiles(files []*ast.File) {
 	for i, file := range files {
 		switch name := file.Name.Name; pkg.name {
 		case "":
-			pkg.name = name
+			if name != "_" {
+				pkg.name = name
+			} else {
+				check.errorf(file.Name.Pos(), "invalid package name _")
+			}
 			fallthrough
 
 		case name:

go/types/check_test.go

@@ -79,6 +79,7 @@ var tests = [][]string{
 	{"testdata/gotos.src"},
 	{"testdata/labels.src"},
 	{"testdata/issues.src"},
+	{"testdata/blank.src"},
 }
 
 var fset = token.NewFileSet()

go/types/package.go

@@ -16,9 +16,13 @@ type Package struct {
 	fake     bool // scope lookup errors are silently dropped if package is fake (internal use only)
 }
 
-// NewPackage returns a new Package for the given package path and name.
+// NewPackage returns a new Package for the given package path and name;
+// the name must not be the blank identifier.
 // The package is not complete and contains no explicit imports.
 func NewPackage(path, name string) *Package {
+	if name == "_" {
+		panic("invalid package name _")
+	}
 	scope := NewScope(Universe, fmt.Sprintf("package %q", path))
 	return &Package{path: path, name: name, scope: scope}
 }

go/types/testdata/blank.src

@@ -0,0 +1,5 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package _ /* ERROR invalid package name */