[dev.go2go] go/types: fix instantiation crash for generic type parameters

Fixes #40038.

src/go/types/fixedbugs/issue40038.go2

@@ -0,0 +1,15 @@
+// Copyright 2020 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 A[type T] int
+
+func (A[T]) m(A[T])
+
+func f[type P interface{m(P)}]()
+
+func _() {
+	_ = f[A]
+}

src/go/types/lookup.go

@@ -396,7 +396,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// In order to compare the signatures, substitute the receiver
 		// type parameters of ftyp with V's instantiation type arguments.
 		// This lazily instantiates the signature of method f.
-		if Vn != nil && len(Vn.targs) > 0 {
+		if Vn != nil && len(Vn.tparams) > 0 {
 			// Be careful: The number of type arguments may not match
 			// the number of receiver parameters. If so, an error was
 			// reported earlier but the length discrepancy is still