[dev.typeparams] go/types: don't permit method calls on ptr to type parameter receivers

This is a port of CL 332609 to go/types.

Change-Id: I3482ea1b97bc7101b987ef312cd6cade80a5843f
Reviewed-on: https://go-review.googlesource.com/c/go/+/335112
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/lookup.go

@@ -75,11 +75,13 @@ func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 
 	typ, isPtr := deref(T)
 
-	// *typ where typ is an interface has no methods.
+	// *typ where typ is an interface or type parameter has no methods.
-	// Be cautious: typ may be nil (issue 39634, crash #3).
+	switch under(typ).(type) {
-	if typ == nil || isPtr && IsInterface(typ) {
+	case *Interface, *TypeParam:
+		if isPtr {
 			return
 		}
+	}
 
 	// Start with typ as single entry at shallowest depth.
 	current := []embeddedType{{typ, nil, isPtr, false}}

src/go/types/testdata/check/issues.go2

@@ -24,22 +24,20 @@ func _() {
 	eql[io.Reader](nil, nil)
 }
 
-// If we have a receiver of pointer type (below: *T) we must ignore
+// If we have a receiver of pointer to type parameter type (below: *T)
-// the pointer in the implementation of the method lookup because
+// we don't have any methods, like for interfaces.
-// the type bound of T is an interface and pointer to interface types
-// have no methods and then the lookup would fail.
 type C[T any] interface {
     m()
 }
 
 // using type bound C
 func _[T C[T]](x *T) {
-	x.m()
+	x.m  /* ERROR x\.m undefined */ ()
 }
 
 // using an interface literal as bound
 func _[T interface{ m() }](x *T) {
-	x.m()
+	x.m  /* ERROR x\.m undefined */ ()
 }
 
 // In a generic function body all method calls will be pointer method calls.

src/go/types/testdata/fixedbugs/issue47031.go2

@@ -0,0 +1,20 @@
+// 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 p
+
+type Mer interface { M() }
+
+func F[T Mer](p *T) {
+	p.M /* ERROR p\.M undefined */ ()
+}
+
+type MyMer int
+
+func (MyMer) M() {}
+
+func _() {
+	F(new(MyMer))
+	F[Mer](nil)
+}

src/go/types/type.go

@@ -113,11 +113,6 @@ func asSignature(t Type) *Signature {
 	return op
 }
 
-func asInterface(t Type) *Interface {
-	op, _ := optype(t).(*Interface)
-	return op
-}
-
 func asMap(t Type) *Map {
 	op, _ := optype(t).(*Map)
 	return op
@@ -128,10 +123,15 @@ func asChan(t Type) *Chan {
 	return op
 }
 
-// If the argument to asNamed and asTypeParam is of the respective types
+// If the argument to asInterface, asNamed, or asTypeParam is of the respective type
 // (possibly after expanding an instance type), these methods return that type.
 // Otherwise the result is nil.
 
+func asInterface(t Type) *Interface {
+	op, _ := optype(t).(*Interface)
+	return op
+}
+
 func asNamed(t Type) *Named {
 	e, _ := expand(t).(*Named)
 	return e