c++: allow variadic operator[] for C++23 [PR103460]

wg21.link/p2128 removed "with exactly one parameter" from over.sub
section.  grok_op_properties has for that the last 2 lines in:
    case OVL_OP_FLAG_BINARY:
      if (arity != 2)
        {
          if (operator_code == ARRAY_REF && cxx_dialect >= cxx23)
            break;
but unfortunately it isn't enough, we reject variadic operator[]
earlier.  The following patch accepts variadic operator[] for C++23
too.

2022-03-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103460
	* decl.cc (grok_op_properties): Allow variadic operator[] for
	C++23.

	* g++.dg/cpp23/subscript7.C: New test.

2 files changed, 22 insertions, 1 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 5821090..37d52d8 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -15214,6 +15214,9 @@ grok_op_properties (tree decl, bool complain)
       if (!arg)
 	{
 	  /* Variadic.  */
+	  if (operator_code == ARRAY_REF && cxx_dialect >= cxx23)
+	    break;
+
 	  error_at (loc, "%qD must not have variable number of arguments",
 		    decl);
 	  return false;
@@ -15289,7 +15292,8 @@ grok_op_properties (tree decl, bool complain)
     }
 
   /* There can be no default arguments.  */
-  for (tree arg = argtypes; arg != void_list_node; arg = TREE_CHAIN (arg))
+  for (tree arg = argtypes; arg && arg != void_list_node;
+       arg = TREE_CHAIN (arg))
     if (TREE_PURPOSE (arg))
       {
 	TREE_PURPOSE (arg) = NULL_TREE;
diff --git a/gcc/testsuite/g++.dg/cpp23/subscript7.C b/gcc/testsuite/g++.dg/cpp23/subscript7.C
new file mode 100644
index 0000000..bb81197
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/subscript7.C
@@ -0,0 +1,17 @@
+// PR c++/103460
+// { dg-do compile }
+// { dg-options "-std=c++23" }
+
+struct S {
+  int &operator[] (int, ...);
+} s;
+struct T {
+  int &operator[] (auto...);
+} t;
+struct U {
+  int &operator[] (...);
+} u;
+
+int a = s[1] + s[2, 1] + s[3, 2, 1] + s[4, 3, 2, 1]
+	+ t[0.0] + t[nullptr, s, 42]
+	+ u[] + u[42] + u[1.5L, 1LL];