[dev.typeparams] cmd/compile/internal/types2: don't permit method calls on ptr to type parameter receivers

Simplify the implementation of asInterface while at it.

For #47031.

Change-Id: Ie7d4fbbab898d961ed3c0b7772ba9604641be13f
Reviewed-on: https://go-review.googlesource.com/c/go/+/332609
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/lookup.go

@@ -73,10 +73,12 @@ func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 
 	typ, isPtr := deref(T)
 
-	// *typ where typ is an interface has no methods.
-	// Be cautious: typ may be nil (issue 39634, crash #3).
-	if typ == nil || isPtr && IsInterface(typ) {
-		return
+	// *typ where typ is an interface or type parameter has no methods.
+	switch under(typ).(type) {
+	case *Interface, *TypeParam:
+		if isPtr {
+			return
+		}
 	}
 
 	// Start with typ as single entry at shallowest depth.

src/cmd/compile/internal/types2/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
-// the pointer in the implementation of the method lookup because
-// the type bound of T is an interface and pointer to interface types
-// have no methods and then the lookup would fail.
+// If we have a receiver of pointer to type parameter type (below: *T)
+// we don't have any methods, like for interfaces.
 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 */ ()
 }
 
 func f2[_ interface{ m1(); m2() }]()

src/cmd/compile/internal/types2/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/cmd/compile/internal/types2/type.go

@@ -112,11 +112,6 @@ 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
@@ -127,10 +122,16 @@ 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.
 
+// asInterface does not need to look at optype (type sets don't contain interfaces)
+func asInterface(t Type) *Interface {
+	u, _ := under(t).(*Interface)
+	return u
+}
+
 func asNamed(t Type) *Named {
 	e, _ := expand(t).(*Named)
 	return e