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>

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

@@ -578,11 +578,7 @@ func transformDot(n *ir.SelectorExpr, isCall bool) ir.Node {
 
 	if (n.Op() == ir.ODOTINTER || n.Op() == ir.ODOTMETH) && !isCall {
 		n.SetOp(ir.OMETHVALUE)
-		if len(n.X.Type().RParams()) > 0 || n.X.Type().IsPtr() && len(n.X.Type().Elem().RParams()) > 0 {
-			// TODO: MethodValueWrapper needed for generics?
-			// Or did we successfully desugar all that at stencil time?
-			return n
-		}
+		// This converts a method type to a function type. See issue 47775.
 		n.SetType(typecheck.NewMethodType(n.Type(), nil))
 	}
 	return n

src/cmd/compile/internal/typecheck/dcl.go

@@ -479,6 +479,12 @@ func autotmpname(n int) string {
 // f is method type, with receiver.
 // return function type, receiver as first argument (or not).
 func NewMethodType(sig *types.Type, recv *types.Type) *types.Type {
+	if sig.HasTParam() {
+		base.Fatalf("NewMethodType with type parameters in signature %+v", sig)
+	}
+	if recv != nil && recv.HasTParam() {
+		base.Fatalf("NewMethodType with type parameters in receiver %+v", recv)
+	}
 	nrecvs := 0
 	if recv != nil {
 		nrecvs++

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{}) {
+}

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]()
+}

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

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]()
+}