cmd/compile: fix irgen mis-handling of ... argument when creating closure

When bulding formal arguments of newly created closure, irgen forgets to
set "..." field attribute, causing type mismatched between the closure
function and the ONAME node represents that closure function.

Fixes #49432

Change-Id: Ieddaa64980cdd3d8cea236a5a9de0204ee21ee39
Reviewed-on: https://go-review.googlesource.com/c/go/+/361961
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

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

@@ -2085,6 +2085,7 @@ func startClosure(pos src.XPos, outer *ir.Func, typ *types.Type) (*ir.Func, []*t
 		fn.Dcl = append(fn.Dcl, arg)
 		f := types.NewField(pos, arg.Sym(), t)
 		f.Nname = arg
+		f.SetIsDDD(typ.Params().Field(i).IsDDD())
 		formalParams = append(formalParams, f)
 	}
 	for i := 0; i < typ.NumResults(); i++ {

test/typeparam/issue49432.go

@@ -0,0 +1,22 @@
+// compile -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
+
+type Handler func(in ...interface{})
+
+type Foo[T any] struct{}
+
+func (b *Foo[T]) Bar(in ...interface{}) {}
+
+func (b *Foo[T]) Init() {
+	_ = Handler(b.Bar)
+}
+
+func main() {
+	c := &Foo[int]{}
+	c.Init()
+}