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

This reverts commit cb6e0639.

The fix is incorrect as it's perfectly fine to refer to an
identifier 'init' inside a function, and 'init' may even be
a variable of function value. Misspelling 'init' in that
context would lead to an incorrect error message.

Reopened #8481.

Change-Id: I49787fdf7738213370ae6f0cab54013e9e3394a8
Reviewed-on: https://go-review.googlesource.com/37876
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3667,19 +3667,11 @@ func typecheckdef(n *Node) *Node {
 				lineno = n.Pos
 			}
 
-			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 "cannot refer to init functions"
+	init()         // ERROR "undefined.*init"
 	runtime.init() // ERROR "unexported.*runtime\.init"
-	var _ = init   // ERROR "cannot refer to init functions"
+	var _ = init   // ERROR "undefined.*init"
 }