From 3c375f1b7ed54471c5323420a15b9bfe162a2527 Mon Sep 17 00:00:00 2001
From: Matthew Dempsky <mdempsky@google.com>
Date: Wed, 22 Nov 2017 09:43:52 -0800
Subject: [PATCH] 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 | 10 ++++++----
 src/go/types/check_test.go         |  1 +
 src/go/types/resolver.go           |  7 +++++++
 src/go/types/testdata/decls5.src   | 10 ++++++++++
 test/fixedbugs/issue21256.go       |  9 +++++++++
 5 files changed, 33 insertions(+), 4 deletions(-)
 create mode 100644 src/go/types/testdata/decls5.src
 create mode 100644 test/fixedbugs/issue21256.go

diff --git a/src/cmd/compile/internal/gc/dcl.go b/src/cmd/compile/internal/gc/dcl.go
index b39bdb5aa03..7d963864570 100644
--- a/src/cmd/compile/internal/gc/dcl.go
+++ b/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 {
diff --git a/src/go/types/check_test.go b/src/go/types/check_test.go
index 24b3365717b..97e224f870f 100644
--- a/src/go/types/check_test.go
+++ b/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"},
diff --git a/src/go/types/resolver.go b/src/go/types/resolver.go
index ba75a0dc239..7bcfaabcde0 100644
--- a/src/go/types/resolver.go
+++ b/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)))
diff --git a/src/go/types/testdata/decls5.src b/src/go/types/testdata/decls5.src
new file mode 100644
index 00000000000..88d31946da4
--- /dev/null
+++ b/src/go/types/testdata/decls5.src
@@ -0,0 +1,10 @@
+// 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
diff --git a/test/fixedbugs/issue21256.go b/test/fixedbugs/issue21256.go
new file mode 100644
index 00000000000..3d3612478df
--- /dev/null
+++ b/test/fixedbugs/issue21256.go
@@ -0,0 +1,9 @@
+// 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"