cmd/compile: fix stencil call expression

In CL 349613,we have supported types.IdentityStrict() that does strict
type comparison.
Therefore, OCONVNOP becomes a possible case in call.X.Op().

Fixes #48604

Change-Id: Ibab27ffcf09656e3380314662f05f38294c1c6ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/351857
Trust: Dan Scales <danscales@google.com>
Trust: David Chase <drchase@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

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

@@ -1093,6 +1093,9 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			case ir.OCALL, ir.OCALLFUNC, ir.OCALLMETH, ir.OCALLINTER:
 				transformCall(call)
 
+			case ir.OCONVNOP:
+				transformCall(call)
+
 			case ir.OFUNCINST:
 				// A call with an OFUNCINST will get transformed
 				// in stencil() once we have created & attached the

test/typeparam/issue48604.go

@@ -0,0 +1,25 @@
+// build -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
+
+type Foo[T any] interface {
+	CreateBar() Bar[T]
+}
+
+type Bar[T any] func() Bar[T]
+
+func (f Bar[T]) CreateBar() Bar[T] {
+	return f
+}
+
+func abc[R any]() {
+	var _ Foo[R] = Bar[R](nil)()
+}
+
+func main() {
+	abc[int]()
+}