From d014fca6d4924b15f0eccb3c8ca6a7f968d88f40 Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Wed, 1 Jul 2020 13:26:41 -0700
Subject: [PATCH] [dev.go2go] go/types: fix assertion failure, better error
 message for inference failure

Fixes #39976.

Change-Id: Idc304a615193847337438e7024e43140472e7bae
Reviewed-on: https://go-review.googlesource.com/c/go/+/240718
Reviewed-by: Robert Griesemer <gri@golang.org>
---
 src/go/types/check_test.go            |  1 +
 src/go/types/fixedbugs/issue39976.go2 | 16 +++++++++
 src/go/types/infer.go                 | 50 +++++++++++++++++++++++++--
 src/go/types/subst.go                 |  7 ++--
 4 files changed, 69 insertions(+), 5 deletions(-)
 create mode 100644 src/go/types/fixedbugs/issue39976.go2

diff --git a/src/go/types/check_test.go b/src/go/types/check_test.go
index 6e3004528d..d750cbdaa8 100644
--- a/src/go/types/check_test.go
+++ b/src/go/types/check_test.go
@@ -136,6 +136,7 @@ var tests = [][]string{
 	{"fixedbugs/issue39768.go2"},
 	{"fixedbugs/issue39938.go2"},
 	{"fixedbugs/issue39948.go2"},
+	{"fixedbugs/issue39976.go2"},
 }
 
 var fset = token.NewFileSet()
diff --git a/src/go/types/fixedbugs/issue39976.go2 b/src/go/types/fixedbugs/issue39976.go2
new file mode 100644
index 0000000000..2413ebfbae
--- /dev/null
+++ b/src/go/types/fixedbugs/issue39976.go2
@@ -0,0 +1,16 @@
+// 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 policy(type K, V) interface{}
+type LRU(type K, V) struct{}
+
+func NewCache(type K, V)(p policy(K, V))
+
+func _() {
+	var lru LRU(int, string)
+	NewCache(int, string)(&lru)
+	NewCache(& /* ERROR does not match policy\(K, V\) \(cannot infer K and V\) */ lru)
+}
diff --git a/src/go/types/infer.go b/src/go/types/infer.go
index d187241c0d..1f5abc377f 100644
--- a/src/go/types/infer.go
+++ b/src/go/types/infer.go
@@ -7,7 +7,10 @@
 
 package types
 
-import "go/token"
+import (
+	"go/token"
+	"strings"
+)
 
 // infer returns the list of actual type arguments for the given list of type parameters tparams
 // by inferring them from the actual arguments args for the parameters params. If infer fails to
@@ -20,7 +23,23 @@ func (check *Checker) infer(pos token.Pos, tparams []*TypeName, params *Tuple, a
 
 	errorf := func(kind string, tpar, targ Type, arg *operand) {
 		// provide a better error message if we can
-		targs, _ := u.x.types()
+		targs, failed := u.x.types()
+		if failed == 0 {
+			// The first type parameter couldn't be inferred.
+			// If none of them could be inferred, don't try
+			// to provide the inferred type in the error msg.
+			allFailed := true
+			for _, targ := range targs {
+				if targ != nil {
+					allFailed = false
+					break
+				}
+			}
+			if allFailed {
+				check.errorf(arg.pos(), "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeNamesString(tparams))
+				return
+			}
+		}
 		smap := makeSubstMap(tparams, targs)
 		inferred := check.subst(arg.pos(), tpar, smap)
 		if inferred != tpar {
@@ -108,6 +127,33 @@ func (check *Checker) infer(pos token.Pos, tparams []*TypeName, params *Tuple, a
 	return targs
 }
 
+// typeNamesString produces a string containing all the
+// type names in list suitable for human consumption.
+func typeNamesString(list []*TypeName) string {
+	// common cases
+	n := len(list)
+	switch n {
+	case 0:
+		return ""
+	case 1:
+		return list[0].name
+	case 2:
+		return list[0].name + " and " + list[1].name
+	}
+
+	// general case (n > 2)
+	var b strings.Builder
+	for i, tname := range list[:n-1] {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		b.WriteString(tname.name)
+	}
+	b.WriteString(", and ")
+	b.WriteString(list[n-1].name)
+	return b.String()
+}
+
 // IsParameterized reports whether typ contains any type parameters.
 // TODO(gri) This is not strictly correct. We only want the free
 // type parameters for a given type. (At the moment, the only way
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 47622b7c39..587e35ba2b 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -22,6 +22,8 @@ type substMap struct {
 	proj  map[*TypeParam]Type
 }
 
+// makeSubstMap creates a new substitution map mapping tpars[i] to targs[i].
+// If targs[i] is nil, tpars[i] is not substituted.
 func makeSubstMap(tpars []*TypeName, targs []Type) *substMap {
 	assert(len(tpars) == len(targs))
 	proj := make(map[*TypeParam]Type, len(tpars))
@@ -29,10 +31,9 @@ func makeSubstMap(tpars []*TypeName, targs []Type) *substMap {
 		// We must expand type arguments otherwise *Instance
 		// types end up as components in composite types.
 		// TODO(gri) explain why this causes problems, if it does
-		targ := expand(targs[i])
+		targ := expand(targs[i]) // possibly nil
 		targs[i] = targ
-		assert(targ != nil)
-		proj[tpar.typ.(*TypeParam)] = targs[i]
+		proj[tpar.typ.(*TypeParam)] = targ
 	}
 	return &substMap{targs, proj}
 }