cmd/compile: stenciled conversions might be NOPs

A generic conversion might be required for when converting T->interface{}.
When stenciled with T=interface{}, then that conversion doesn't need
to do anything.

Fixes #48276

Change-Id: Ife65d01c99fbd0895cb7eec79df9e93e752b1fa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/348736
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

Author: randall77

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

@@ -1177,6 +1177,12 @@ func (subst *subster) node(n ir.Node) ir.Node {
 
 		case ir.OCONVIFACE:
 			x := x.(*ir.ConvExpr)
+			if m.Type().IsEmptyInterface() && m.(*ir.ConvExpr).X.Type().IsEmptyInterface() {
+				// Was T->interface{}, after stenciling it is now interface{}->interface{}.
+				// No longer need the conversion. See issue 48276.
+				m.(*ir.ConvExpr).SetOp(ir.OCONVNOP)
+				break
+			}
 			// Note: x's argument is still typed as a type parameter.
 			// m's argument now has an instantiated type.
 			if x.X.Type().HasTParam() || (x.X.Type().IsInterface() && x.Type().HasTParam()) {

test/typeparam/issue48276a.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
+
+import "fmt"
+
+func main() {
+	IsZero[interface{}]("")
+}
+
+func IsZero[T comparable](val T) bool {
+	var zero T
+	fmt.Printf("%v:%v\n", zero, val)
+	return val != zero
+}

test/typeparam/issue48276a.out

@@ -0,0 +1 @@
+<nil>:

test/typeparam/issue48276b.go

@@ -0,0 +1,15 @@
+// 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 main() {
+	f[interface{}](nil)
+}
+
+func f[T any](x T) {
+	var _ interface{} = x
+}