cmd/compile: import empty closure function correctly

On import, make sure that an empty closure is represented as a single
empty block statement. Otherwise, the closure is dropped. Block
statements are not exported explicitly, so must recreate on import.

Fixes #44330

Change-Id: I061598f0f859dd71d2d0cbd10c77cdd81525d1f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/297569
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>

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

@@ -992,6 +992,11 @@ func (r *importReader) node() ir.Node {
 		r.funcBody(fn)
 		fn.Dcl = fn.Inl.Dcl
 		fn.Body = fn.Inl.Body
+		if len(fn.Body) == 0 {
+			// An empty closure must be represented as a single empty
+			// block statement, else it will be dropped.
+			fn.Body = []ir.Node{ir.NewBlockStmt(src.NoXPos, nil)}
+		}
 		fn.Inl = nil
 
 		ir.FinishCaptureNames(pos, r.curfn, fn)

test/fixedbugs/issue44330.dir/a.go

@@ -0,0 +1,21 @@
+// Copyright 2021 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 a
+
+type Table struct {
+	ColumnSeparator bool
+	RowSeparator    bool
+
+	// ColumnResizer is called on each Draw. Can be used for custom column sizing.
+	ColumnResizer func()
+}
+
+func NewTable() *Table {
+	return &Table{
+		ColumnSeparator: true,
+		RowSeparator:    true,
+		ColumnResizer:   func() {},
+	}
+}

test/fixedbugs/issue44330.dir/b.go

@@ -0,0 +1,23 @@
+// Copyright 2021 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
+
+import (
+	"./a"
+)
+
+type Term struct {
+	top *a.Table
+}
+
+//go:noinline
+func NewFred() *Term {
+	table := a.NewTable()
+	return &Term{top: table}
+}
+
+func main() {
+	NewFred()
+}

test/fixedbugs/issue44330.go

@@ -0,0 +1,7 @@
+// rundir
+
+// Copyright 2021 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 ignored