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>

diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index 1c22fc2ac0772ca72ff11462b28a3bb9a1ec8715..a524ddc2a086f38ad1fa046abcfcde3efe1a24f0 100644 (file)
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/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()) {

diff --git a/test/typeparam/issue48276a.go b/test/typeparam/issue48276a.go
new file mode 100644 (file)
index 0000000..060ac3e
--- /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 (file)
index 0000000..7e8a8a9
--- /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 (file)
index 0000000..67c3e3d
--- /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
+}