cmd/compile: fix const declaration group broken with closure

In typecheckclosure, a xfunc node will be put to xtop. But that node can
be shared between multiple closures, like in a const declaration group:

const (
	x = unsafe.Sizeof(func() {})
	y
)

It makes a xfunc node appears multiple times in xtop, causing duplicate
initLSym run.

To fix this issue, we only do typecheck for xfunc one time, and setup
closure node earlier in typecheckclosure process.

Fixes #30709

Change-Id: Ic924a157ee9f3e5d776214bef5390849ddc8aab9
Reviewed-on: https://go-review.googlesource.com/c/go/+/172298
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/closure.go

@@ -73,6 +73,16 @@ func (p *noder) funcLit(expr *syntax.FuncLit) *Node {
 
 func typecheckclosure(clo *Node, top int) {
 	xfunc := clo.Func.Closure
+	clo.Func.Ntype = typecheck(clo.Func.Ntype, Etype)
+	clo.Type = clo.Func.Ntype.Type
+	clo.Func.Top = top
+
+	// Do not typecheck xfunc twice, otherwise, we will end up pushing
+	// xfunc to xtop multiple times, causing initLSym called twice.
+	// See #30709
+	if xfunc.Typecheck() == 1 {
+		return
+	}
 
 	for _, ln := range xfunc.Func.Cvars.Slice() {
 		n := ln.Name.Defn
@@ -95,10 +105,6 @@ func typecheckclosure(clo *Node, top int) {
 	declare(xfunc.Func.Nname, PFUNC)
 	xfunc = typecheck(xfunc, ctxStmt)
 
-	clo.Func.Ntype = typecheck(clo.Func.Ntype, Etype)
-	clo.Type = clo.Func.Ntype.Type
-	clo.Func.Top = top
-
 	// Type check the body now, but only if we're inside a function.
 	// At top level (in a variable initialization: curfn==nil) we're not
 	// ready to type check code yet; we'll check it later, because the

test/fixedbugs/issue30709.go

@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2019 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.
+
+// Check closure in const declaration group can be compiled
+// and set correct value
+
+package main
+
+import "unsafe"
+
+const (
+	x = unsafe.Sizeof(func() {})
+	y
+)
+
+func main() {
+	const (
+		z = unsafe.Sizeof(func() {})
+		t
+	)
+
+	// x and y must be equal
+	println(x == y)
+	// size must be greater than zero
+	println(y > 0)
+
+	// Same logic as x, y above
+	println(z == t)
+	println(t > 0)
+}

test/fixedbugs/issue30709.out

@@ -0,0 +1,4 @@
+true
+true
+true
+true