cmd/compile: always remove receiver type from instantiated method values

If a type T has a method foo, then

var t T
var i interface{} = t.foo

The type of foo is a method type, but the type of t.foo should be a
standard function type. Make sure we always do that conversion.

Fixes #47775

Change-Id: I464ec792196b050aba1914e070a4ede34bfd0bfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/343881
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dan Scales <danscales@google.com>

4 files changed, 65 insertions, 0 deletions

diff --git a/test/typeparam/issue47775.dir/b.go b/test/typeparam/issue47775.dir/b.go
new file mode 100644
index 0000000000..b6d7ba97c5
--- /dev/null
+++ b/test/typeparam/issue47775.dir/b.go
@@ -0,0 +1,19 @@
+// 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 b
+
+type C[T any] struct {
+}
+
+func (c *C[T]) reset() {
+}
+
+func New[T any]() {
+	c := &C[T]{}
+	z(c.reset)
+}
+
+func z(interface{}) {
+}
diff --git a/test/typeparam/issue47775.dir/main.go b/test/typeparam/issue47775.dir/main.go
new file mode 100644
index 0000000000..ed284ddfc6
--- /dev/null
+++ b/test/typeparam/issue47775.dir/main.go
@@ -0,0 +1,11 @@
+// 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 "b"
+
+func main() {
+	b.New[int]()
+}
diff --git a/test/typeparam/issue47775.go b/test/typeparam/issue47775.go
new file mode 100644
index 0000000000..76930e5e4f
--- /dev/null
+++ b/test/typeparam/issue47775.go
@@ -0,0 +1,7 @@
+// rundir -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 ignored
diff --git a/test/typeparam/issue47775b.go b/test/typeparam/issue47775b.go
new file mode 100644
index 0000000000..6d3fc8df97
--- /dev/null
+++ b/test/typeparam/issue47775b.go
@@ -0,0 +1,28 @@
+// 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
+
+type C[T any] struct {
+}
+
+func (c *C[T]) reset() {
+}
+
+func New[T any]() {
+	c := &C[T]{}
+	i = c.reset
+	z(c.reset)
+}
+
+var i interface{}
+
+func z(interface{}) {
+}
+
+func main() {
+	New[int]()
+}