go/types: don't panic when checking a ListExpr in exprInternal

As an alternative to CL 312149, add a catch-all error message in
exprInternal when encountering a ListExpr, rather than panicking.

We still might want something like CL 312149 to improve the error
message or recovery from bad indexing.

Change-Id: I865f7cc4eefa4a3b7bd8f3100df96d0144e1712f
Reviewed-on: https://go-review.googlesource.com/c/go/+/313909
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/internal/typeparams/notypeparams.go

@@ -30,6 +30,10 @@ func UnpackExpr(expr ast.Expr) []ast.Expr {
 	return []ast.Expr{expr}
 }
 
+func IsListExpr(n ast.Node) bool {
+	return false
+}
+
 func Get(ast.Node) *ast.FieldList {
 	return nil
 }

src/go/internal/typeparams/typeparams.go

@@ -38,6 +38,11 @@ func UnpackExpr(x ast.Expr) []ast.Expr {
 	return nil
 }
 
+func IsListExpr(n ast.Node) bool {
+	_, ok := n.(*ast.ListExpr)
+	return ok
+}
+
 func Get(n ast.Node) *ast.FieldList {
 	switch n := n.(type) {
 	case *ast.TypeSpec:

src/go/types/expr.go

@@ -1805,7 +1805,11 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		// types, which are comparatively rare.
 
 	default:
-		panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
+		if typeparams.IsListExpr(e) {
+			check.errorf(e, _Todo, "invalid multi-index expression")
+		} else {
+			panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
+		}
 	}
 
 	// everything went well

src/go/types/fixedbugs/issue45635.go2

@@ -7,3 +7,26 @@ package main
 func main() {
 	some /* ERROR "undeclared name" */ [int, int]()
 }
+
+type N[T any] struct{}
+
+var _ N /* ERROR "0 arguments but 1 type parameters" */ []
+
+type I interface {
+	type map[int]int, []int
+}
+
+func _[T I]() {
+	var m map[int]int
+	_ = m[1 /* ERROR "multi-index expression" */, 2 /* ERROR "expected type" */ ]
+
+	var a [3]int
+	_ = a[1 /* ERROR "multi-index expression" */, 2 /* ERROR "expected type" */ ]
+
+	var s []int
+	_ = s[1 /* ERROR "multi-index expression" */, 2 /* ERROR "expected type" */ ]
+
+	var t T
+	// TODO(rFindley) Fix the duplicate error below.
+	_ = t[1 /* ERROR "multi-index expression" */ /* ERROR "multi-index expression" */, 2 /* ERROR "expected type" */ ]
+}