go/types: fix panic when using multiple type arguments

Fix a panic caused by using type arguments without first unpacking.

This was noticed in the review of CL 300998, but unfortunately not yet
fixed.

Fixes #45635

Change-Id: I8ab1720f3e27a6002bc925f0eea943ec6f778341
Reviewed-on: https://go-review.googlesource.com/c/go/+/311669
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/expr.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/constant"
+	"go/internal/typeparams"
 	"go/token"
 	"math"
 )
@@ -1435,7 +1436,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 	case *ast.IndexExpr:
 		check.exprOrType(x, e.X)
 		if x.mode == invalid {
-			check.use(e.Index)
+			check.use(typeparams.UnpackExpr(e.Index)...)
 			goto Error
 		}
 

src/go/types/fixedbugs/issue45635.go2

@@ -0,0 +1,9 @@
+// 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 main
+
+func main() {
+	some /* ERROR "undeclared name" */ [int, int]()
+}