[dev.go2go] go/types: don't crash when receiver type doesn't declare type parameters

Fixes #39664.

Change-Id: I0cf585f0c704bdaa163a7ebc9e7c82a734fcd5e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/238625
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/check_test.go

@@ -120,6 +120,11 @@ var tests = [][]string{
 	{"examples/functions.go2"},
 	{"examples/methods.go2"},
 	{"examples/types.go2"},
+
+	// Go 2 prototype bug fixes
+	// TODO(gri) Eliminate the need to enumerate these tests here.
+	//           Should just traverse that directory.
+	{"fixedbugs/issue39664.go2"},
 }
 
 var fset = token.NewFileSet()

src/go/types/fixedbugs/issue39664.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 T(type _) struct {}
+
+func (T /* ERROR instantiation */ ) m()
+
+func _() {
+	var x interface { m() }
+	x = T(int){}
+	_ = x
+}

src/go/types/lookup.go

@@ -396,6 +396,15 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// 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 {
+			// 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
+			// here. Exit early in this case to prevent an assertion
+			// failure in makeSubstMap.
+			// TODO(gri) Can we avoid this check by fixing the lengths?
+			if len(ftyp.rparams) != len(Vn.targs) {
+				return
+			}
 			ftyp = check.subst(token.NoPos, ftyp, makeSubstMap(ftyp.rparams, Vn.targs)).(*Signature)
 		}