cmd/compile, go/types: error if main.main is not a function

Fixes #21256.

Change-Id: I3af4c76e734c09d07f15525b793a544a7279b906
Reviewed-on: https://go-review.googlesource.com/79435
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -85,12 +85,14 @@ func declare(n *Node, ctxt Class) {
 		yyerror("cannot declare name %v", s)
 	}
 
-	if ctxt == PEXTERN && s.Name == "init" {
-		yyerror("cannot declare init - must be func")
-	}
-
 	gen := 0
 	if ctxt == PEXTERN {
+		if s.Name == "init" {
+			yyerror("cannot declare init - must be func")
+		}
+		if s.Name == "main" && localpkg.Name == "main" {
+			yyerror("cannot declare main - must be func")
+		}
 		externdcl = append(externdcl, n)
 	} else {
 		if Curfn == nil && ctxt == PAUTO {

src/go/types/check_test.go

@@ -69,6 +69,7 @@ var tests = [][]string{
 	{"testdata/decls2a.src", "testdata/decls2b.src"},
 	{"testdata/decls3.src"},
 	{"testdata/decls4.src"},
+	{"testdata/decls5.src"},
 	{"testdata/const0.src"},
 	{"testdata/const1.src"},
 	{"testdata/constdecl.src"},

src/go/types/resolver.go

@@ -111,6 +111,13 @@ func (check *Checker) declarePkgObj(ident *ast.Ident, obj Object, d *declInfo) {
 		return
 	}
 
+	// spec: "The main package must have package name main and declare
+	// a function main that takes no arguments and returns no value."
+	if ident.Name == "main" && check.pkg.name == "main" {
+		check.errorf(ident.Pos(), "cannot declare main - must be func")
+		return
+	}
+
 	check.declare(check.pkg.scope, ident, obj, token.NoPos)
 	check.objMap[obj] = d
 	obj.setOrder(uint32(len(check.objMap)))

src/go/types/testdata/decls5.src

+// Copyright 2017 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 main
+
+// declarations of main
+const _, main /* ERROR "cannot declare main" */ , _ = 0, 1, 2
+type main /* ERROR "cannot declare main" */ struct{}
+var _, main /* ERROR "cannot declare main" */ int

test/fixedbugs/issue21256.go

+// errorcheck
+
+// Copyright 2017 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 main
+
+var main = func() {} // ERROR "must be func"