cmd/compile: improve error message if init is directly invoked

Fixes #8481.

Inform the user that init functions cannot be directly invoked
in user code, as mandated by the spec at:
http://golang.org/ref/spec#Program_initialization_and_execution.

Change-Id: Ib12c0c08718ffd48b76b6f9b13c76bb6612d2e7b
Reviewed-on: https://go-review.googlesource.com/34790Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -3648,11 +3648,19 @@ func typecheckdef(n *Node) *Node {
 				lineno = n.Pos
 			}
 
-			// Note: adderrorname looks for this string and
-			// adds context about the outer expression
-			yyerror("undefined: %v", n.Sym)
+			switch n.Sym.Name {
+			case "init":
+				// As per the spec at:
+				//  https://golang.org/ref/spec#Program_initialization_and_execution
+				// init cannot be referred to in usercode.
+				// See https://golang.org/issues/8481.
+				yyerror("cannot refer to init functions")
+			default:
+				// Note: adderrorname looks for this string and
+				// adds context about the outer expression
+				yyerror("undefined: %v", n.Sym)
+			}
 		}
-
 		return n
 	}
 

test/fixedbugs/issue8481.go

+// errorcheck
+
+// Copyright 2016 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
+
+func init() {
+}
+
+func main() {
+	init() // ERROR "cannot refer to init functions"
+}

test/init.go

@@ -15,7 +15,7 @@ func init() {
 }
 
 func main() {
-	init()         // ERROR "undefined.*init"
+	init()         // ERROR "cannot refer to init functions"
 	runtime.init() // ERROR "unexported.*runtime\.init"
-	var _ = init   // ERROR "undefined.*init"
+	var _ = init   // ERROR "cannot refer to init functions"
 }