[dev.regabi] go/types: report error for invalid main function signature

This is a port of CL 279424, which didn't make it into master in time
for go1.16. Move it to dev.regabi so that it may be merged.

Notably, this port no longer removes the _InvalidInitSig error code,
instead opting to deprecate it. Now that error codes are 'locked in' for
go1.16, even if their API may not yet be exposed, we should follow the
practice of not changing their values. In the future, code generation
can make it easier to keep error code values constant.

For #43308

Change-Id: I5260b93fd063393d38d6458e45a67e7f9b7426ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/289714
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/decl.go

@@ -751,8 +751,12 @@ func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
 	obj.typ = sig // guard against cycles
 	fdecl := decl.fdecl
 	check.funcType(sig, fdecl.Recv, fdecl.Type)
-	if sig.recv == nil && obj.name == "init" && (sig.params.Len() > 0 || sig.results.Len() > 0) {
-		check.errorf(fdecl, _InvalidInitSig, "func init must have no arguments and no return values")
+	if sig.recv == nil {
+		if obj.name == "init" && (sig.params.Len() > 0 || sig.results.Len() > 0) {
+			check.errorf(fdecl, _InvalidInitDecl, "func init must have no arguments and no return values")
+		} else if obj.name == "main" && check.pkg.name == "main" && (sig.params.Len() > 0 || sig.results.Len() > 0) {
+			check.errorf(fdecl, _InvalidMainDecl, "func main must have no arguments and no return values")
+		}
 		// ok to continue
 	}
 

src/go/types/errorcodes.go

@@ -386,15 +386,18 @@ const (
 	// _InvalidInitSig occurs when an init function declares parameters or
 	// results.
 	//
-	// Example:
-	//  func init() int { return 1 }
+	// Deprecated: no longer emitted by the type checker. _InvalidInitDecl is
+	// used instead.
 	_InvalidInitSig
 
 	// _InvalidInitDecl occurs when init is declared as anything other than a
 	// function.
 	//
 	// Example:
 	//  var init = 1
+	//
+	// Example:
+	//  func init() int { return 1 }
 	_InvalidInitDecl
 
 	// _InvalidMainDecl occurs when main is declared as anything other than a

src/go/types/testdata/main.src

@@ -0,0 +1,9 @@
+// Copyright 2020 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 main()
+func /* ERROR "no arguments and no return values" */ main /* ERROR redeclared */ (int)
+func /* ERROR "no arguments and no return values" */ main /* ERROR redeclared */ () int