cmd/compile: fix implement for closure in a global assignment

If closure in a global assignment and has a method receiver.
We should assign receiver as a global variable, not a local variable.

Fixes #48225

Change-Id: I8f65dd6e8baf66a5eff24028d28ad0a594091add
Reviewed-on: https://go-review.googlesource.com/c/go/+/348512
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/stencil.go

@@ -396,13 +396,19 @@ func (g *irgen) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 	if rcvrValue != nil {
 		rcvrVar = ir.NewNameAt(pos, typecheck.LookupNum(".rcvr", g.dnum))
 		g.dnum++
-		rcvrVar.Class = ir.PAUTO
 		typed(rcvrValue.Type(), rcvrVar)
-		rcvrVar.Curfn = outer
 		rcvrAssign = ir.NewAssignStmt(pos, rcvrVar, rcvrValue)
 		rcvrAssign.SetTypecheck(1)
 		rcvrVar.Defn = rcvrAssign
-		outer.Dcl = append(outer.Dcl, rcvrVar)
+		if outer == nil {
+			rcvrVar.Class = ir.PEXTERN
+			g.target.Decls = append(g.target.Decls, rcvrAssign)
+			g.target.Externs = append(g.target.Externs, rcvrVar)
+		} else {
+			rcvrVar.Class = ir.PAUTO
+			rcvrVar.Curfn = outer
+			outer.Dcl = append(outer.Dcl, rcvrVar)
+		}
 	}
 
 	// Build body of closure. This involves just calling the wrapped function directly

test/typeparam/issue48225.go

@@ -0,0 +1,37 @@
+// 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
+
+import "reflect"
+
+type Foo[T any] struct {
+	val int
+}
+
+func (foo Foo[T]) Get() *T {
+	if foo.val != 1 {
+		panic("bad val field in Foo receiver")
+	}
+	return new(T)
+}
+
+var (
+	newInt    = Foo[int]{val: 1}.Get
+	newString = Foo[string]{val: 1}.Get
+)
+
+func main() {
+	i := newInt()
+	s := newString()
+
+	if t := reflect.TypeOf(i).String(); t != "*int" {
+		panic(t)
+	}
+	if t := reflect.TypeOf(s).String(); t != "*string" {
+		panic(t)
+	}
+}