From 3935b33b310f364aced53ef492df28245b10f986 Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Mon, 9 Mar 2020 21:45:21 -0700
Subject: [PATCH] go/types: avoid follow-on panic after assertion failure in
 subst

Also: Updated NOTES.

Change-Id: Iabc45e46aa76bacf5c6984dab6f4bcab74392562
---
 src/go/types/NOTES            |  3 ++-
 src/go/types/contracts.go     |  3 +++
 src/go/types/subst.go         |  7 ++++++-
 src/go/types/testdata/tmp.go2 | 17 +++++++++++++----
 4 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/src/go/types/NOTES b/src/go/types/NOTES
index 335eb58c5a..ecb80d62e5 100644
--- a/src/go/types/NOTES
+++ b/src/go/types/NOTES
@@ -4,7 +4,7 @@ so we have a better track record. I only switched to this file in Nov 2019, henc
 ----------------------------------------------------------------------------------------------------
 TODO
 
-- use Underlying() to return a type parameter's bound? investigate!
+- review embedding of contracts with multiple and different numbers of arguments - likely incorrect
 - debug (and error msg) printing of generic instantiated types needs some work
 - improve error messages!
 - use []*TypeParam for tparams in subst? (unclear)
@@ -14,6 +14,7 @@ TODO
 ----------------------------------------------------------------------------------------------------
 KNOWN ISSUES
 
+- 'comparable' predeclared contract is not implemented yet
 - iteration over generic variables doesn't report certain channel errors (see TODOs in code)
 - cannot handle mutually recursive parameterized interfaces using themselves as type bounds
 - contract instantiation requires the type arguments to be type parameters from the type of function
diff --git a/src/go/types/contracts.go b/src/go/types/contracts.go
index 817368efb7..818ca05484 100644
--- a/src/go/types/contracts.go
+++ b/src/go/types/contracts.go
@@ -137,6 +137,9 @@ func (check *Checker) contractDecl(obj *Contract, cdecl *ast.ContractSpec) {
 
 				// add the instantiated bounds as embedded interfaces to the respective
 				// embedding (outer) contract bound
+				// TODO(gri) This seems incorrect. We must accomodate the situation where
+				//           the embedded interfaces have different type parameters than
+				//           the embedding interfaces.
 				for i, ebound := range eobj.Bounds {
 					index := targs[i].(*TypeParam).index
 					iface := bounds[index].underlying.(*Interface)
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 2326795e86..7f6427df76 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -73,6 +73,11 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 	case *Signature:
 		tparams = t.tparams
 		defer func() {
+			// If we had an unexpected failure somewhere don't
+			// panic below when asserting res.(*Signature).
+			if res == nil {
+				return
+			}
 			// If the signature doesn't use its type parameters, subst
 			// will not make a copy. In that case, make a copy now (so
 			// we can set tparams to nil w/o causing side-effects).
@@ -80,7 +85,7 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 				copy := *t
 				res = &copy
 			}
-			// After instantiating a generic signure, it is not generic
+			// After instantiating a generic signature, it is not generic
 			// anymore; we need to set tparams to nil.
 			res.(*Signature).tparams = nil
 		}()
diff --git a/src/go/types/testdata/tmp.go2 b/src/go/types/testdata/tmp.go2
index 7255712b55..261cf2cb52 100644
--- a/src/go/types/testdata/tmp.go2
+++ b/src/go/types/testdata/tmp.go2
@@ -2,9 +2,18 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package p
+package main
 
-type T(type a, b, c) struct {}
+contract anInt(T) {
+	T int
+}
 
-//func (_ T(_, T(_, _, _), _)) m() {}
-func (_ T(_, int, _)) m() {}
+contract twoInt(K, _) {
+	anInt(K)
+}
+
+func f(type K, V twoInt)()
+
+func _ () {
+	// f(int, int)() // this fails at the moment
+}
\ No newline at end of file