cmd/compile: fix check to avoid creating new closure function when typechecking inline body

By default, when typechecking a closure, tcClosure() creates a new
closure function. This should really be done separate from typechecking.
For now, we explicitly avoid creating a new closure function when
typechecking an inline body (in ImportedBody). However, the heuristic
for determining when we are typechecking an inline body was not correct
for double nested closures in an inline body, since CurFunc will then be
the inner closure, which has a body.

So, use a simple global variable to indicate when we typechecking an
inline body. The global variable is fine (just like ir.CurFunc), since
the front-end runs serially.

Fixes #44325

Change-Id: If2829fe1ebb195a7b1a240192b57fe6f04d1a36b
Reviewed-on: https://go-review.googlesource.com/c/go/+/294211
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>

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

@@ -100,6 +100,11 @@ func PartialCallType(n *ir.SelectorExpr) *types.Type {
 	return t
 }
 
+// True if we are typechecking an inline body in ImportedBody below. We use this
+// flag to not create a new closure function in tcClosure when we are just
+// typechecking an inline body, as opposed to the body of a real function.
+var inTypeCheckInl bool
+
 // Lazy typechecking of imported bodies. For local functions, CanInline will set ->typecheck
 // because they're a copy of an already checked body.
 func ImportedBody(fn *ir.Func) {
@@ -138,7 +143,12 @@ func ImportedBody(fn *ir.Func) {
 
 	savefn := ir.CurFunc
 	ir.CurFunc = fn
+	if inTypeCheckInl {
+		base.Fatalf("inTypeCheckInl should not be set recursively")
+	}
+	inTypeCheckInl = true
 	Stmts(fn.Inl.Body)
+	inTypeCheckInl = false
 	ir.CurFunc = savefn
 
 	// During ImportBody (which imports fn.Func.Inl.Body),
@@ -307,7 +317,6 @@ func tcClosure(clo *ir.ClosureExpr, top int) {
 	// body in ImportedBody(), since we only want to create the named function
 	// when the closure is actually inlined (and then we force a typecheck
 	// explicitly in (*inlsubst).node()).
-	inTypeCheckInl := ir.CurFunc != nil && ir.CurFunc.Body == nil
 	if !inTypeCheckInl {
 		fn.Nname.SetSym(ClosureName(ir.CurFunc))
 		ir.MarkFunc(fn.Nname)

test/fixedbugs/issue44325.dir/a.go

@@ -0,0 +1,13 @@
+// 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
+
+func FM() func() {
+	return func() {
+		_ = func() int {
+			return 0
+		}
+	}
+}

test/fixedbugs/issue44325.dir/b.go

@@ -0,0 +1,13 @@
+// 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 b
+
+import (
+	"./a"
+)
+
+func F() {
+	a.FM()
+}

test/fixedbugs/issue44325.go

@@ -0,0 +1,7 @@
+// compiledir
+
+// 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