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>

3 files changed, 35 insertions, 0 deletions

diff --git a/test/typeparam/issue48276a.go b/test/typeparam/issue48276a.go
new file mode 100644
index 0000000000..060ac3eb7f
--- /dev/null
+++ b/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
+}
diff --git a/test/typeparam/issue48276a.out b/test/typeparam/issue48276a.out
new file mode 100644
index 0000000000..7e8a8a9a2e
--- /dev/null
+++ b/test/typeparam/issue48276a.out
@@ -0,0 +1 @@
+<nil>:
diff --git a/test/typeparam/issue48276b.go b/test/typeparam/issue48276b.go
new file mode 100644
index 0000000000..67c3e3d9f5
--- /dev/null
+++ b/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
+}