cmd/compile: fix dictionaries for nested closures

Capturing dictionary closure variables is ok.

Fixes #47684

Change-Id: I049c87117915e0c5a172b9665bfac2f91064b2d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/342050
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/ir/name.go

@@ -404,7 +404,9 @@ func CaptureName(pos src.XPos, fn *Func, n *Name) *Name {
 	if n.Op() != ONAME || n.Curfn == nil {
 		return n // okay to use directly
 	}
-	if n.IsClosureVar() {
+	if n.IsClosureVar() && n.Sym().Name != ".dict" {
+		// Note: capturing dictionary closure variables is ok. This makes
+		// sure the generated code is correctly optimized.
 		base.FatalfAt(pos, "misuse of CaptureName on closure variable: %v", n)
 	}
 

test/typeparam/issue47684.go

@@ -0,0 +1,19 @@
+// run -gcflags=-G=3
+
+// 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
+
+func f[G any]() int {
+	return func() int {
+		return func() int {
+			return 0
+		}()
+	}()
+}
+
+func main() {
+	f[int]()
+}

test/typeparam/issue47684b.go

@@ -0,0 +1,23 @@
+// run -gcflags=-G=3
+
+// 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
+
+func f[G any]() interface{} {
+	return func() interface{} {
+		return func() interface{} {
+			var x G
+			return x
+		}()
+	}()
+}
+
+func main() {
+	x := f[int]()
+	if v, ok := x.(int); !ok || v != 0 {
+		panic("bad")
+	}
+}

test/typeparam/issue47684c.go

@@ -0,0 +1,19 @@
+// run -gcflags=-G=3
+
+// 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
+
+func f[G any]() func()func()int {
+	return func() func()int {
+		return func() int {
+			return 0
+		}
+	}
+}
+
+func main() {
+	f[int]()()()
+}