c++: Implement C++23 P2589R1 - - static operator[]

Here is a patch that implements the static operator[] paper.
One thing that doesn't work properly is the same problem as I've filed
yesterday for static operator() - PR107624 - that side-effects of
the postfix-expression on which the call or subscript operator are
applied are thrown away, I assume we have to add them into COMPOUND_EXPR
somewhere after we find out that the we've chosen a static member function
operator.

2022-11-15  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Bump C++23
	__cpp_multidimensional_subscript macro value to 202211L.
gcc/cp/
	* decl.cc (grok_op_properties): Implement C++23 P2589R1
	- static operator[].  Handle operator[] similarly to operator()
	- allow static member functions, but pedwarn on it for C++20 and
	older.  Unlike operator(), perform rest of checks on it though for
	C++20.
	* call.cc (add_operator_candidates): For operator[] with class
	typed first parameter, pass that parameter as first_arg and
	an adjusted arglist without that parameter.
gcc/testsuite/
	* g++.dg/cpp23/subscript9.C: New test.
	* g++.dg/cpp23/feat-cxx2b.C: Expect a newer
	__cpp_multidimensional_subscript value.
	* g++.old-deja/g++.bugs/900210_10.C: Don't expect an error
	for C++23 or later.

1 files changed, 30 insertions, 6 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index ef618d5..dbb01ec 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -6589,12 +6589,36 @@ add_operator_candidates (z_candidate **candidates,
       if (fns == error_mark_node)
 	return error_mark_node;
       if (fns)
-	add_candidates (BASELINK_FUNCTIONS (fns),
-			NULL_TREE, arglist, NULL_TREE,
-			NULL_TREE, false,
-			BASELINK_BINFO (fns),
-			BASELINK_ACCESS_BINFO (fns),
-			flags, candidates, complain);
+	{
+	  if (code == ARRAY_REF)
+	    {
+	      vec<tree,va_gc> *restlist = make_tree_vector ();
+	      for (unsigned i = 1; i < nargs; ++i)
+		vec_safe_push (restlist, (*arglist)[i]);
+	      z_candidate *save_cand = *candidates;
+	      add_candidates (BASELINK_FUNCTIONS (fns),
+			      (*arglist)[0], restlist, NULL_TREE,
+			      NULL_TREE, false,
+			      BASELINK_BINFO (fns),
+			      BASELINK_ACCESS_BINFO (fns),
+			      flags, candidates, complain);
+	      /* Release the vec if we didn't add a candidate that uses it.  */
+	      for (z_candidate *c = *candidates; c != save_cand; c = c->next)
+		if (c->args == restlist)
+		  {
+		    restlist = NULL;
+		    break;
+		  }
+	      release_tree_vector (restlist);
+	    }
+	  else
+	    add_candidates (BASELINK_FUNCTIONS (fns),
+			    NULL_TREE, arglist, NULL_TREE,
+			    NULL_TREE, false,
+			    BASELINK_BINFO (fns),
+			    BASELINK_ACCESS_BINFO (fns),
+			    flags, candidates, complain);
+	}
     }
   /* Per [over.match.oper]3.2, if no operand has a class type, then
      only non-member functions that have type T1 or reference to