c++, v2: Implement C++23 P2128R6 - Multidimensional subscript operator [PR102611]

  On Tue, Nov 23, 2021 at 10:28:48PM -0500, Jason Merrill wrote:

Thanks.

> > +	  while (true)
> > +	    {
> > +	      cp_expr expr (NULL_TREE);
> > +	      /* Parse the next assignment-expression.  */
> > +	      if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
> > +		{
> > +		  /* A braced-init-list.  */
> > +		  bool expr_nonconst_p;
> > +		  cp_lexer_set_source_position (parser->lexer);
> > +		  expr = cp_parser_braced_list (parser, &expr_nonconst_p);
> > +		}
> > +	      else
> > +		expr = cp_parser_assignment_expression (parser);
> > +
> > +	      /* If we have an ellipsis, then this is an expression
> > +		 expansion.  */
> > +	      if (cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
> > +		{
> > +		  /* Consume the `...'.  */
> > +		  cp_lexer_consume_token (parser->lexer);
> > +		  /* Build the argument pack.  */
> > +		  expr = make_pack_expansion (expr);
> > +		}
> > +
> > +	      if (expr == error_mark_node)
> > +		index = error_mark_node;
> > +	      else if (expression_list.get () == NULL
> > +		       && !PACK_EXPANSION_P (expr.get_value ()))
> > +		index = expr.get_value ();
> > +	      else
> > +		vec_safe_push (expression_list, expr.get_value ());
> > +
> > +	      /* If the next token isn't a `,', then we are done.  */
> > +	      if (cp_lexer_next_token_is_not (parser->lexer, CPP_COMMA))
> > +		break;
> > +
> > +	      if (expression_list.get () == NULL && index != error_mark_node)
> > +		{
> > +		  *&expression_list = make_tree_vector_single (index);
> > +		  index = NULL_TREE;
> > +		}
> > +
> > +	      /* Otherwise, consume the `,' and keep going.  */
> > +	      cp_lexer_consume_token (parser->lexer);
> > +	    }
> 
> Let's share this loop with cp_parser_parenthesized_expression_list.

I'd prefer not to share the loop as whole because what exactly is done with
the parsed expressions differs a lot, for the array refs I'd prefer not to
push anything into a vector for the most common case with a single element.
I've outlined into a function what I think I can easily share
(see cp_parser_parenthesized_expression_list_elt in the updated patch).

> > +	  if (expression_list.get () && index == error_mark_node)
> > +	    {
> > +	      release_tree_vector (*&expression_list);
> > +	      *&expression_list = NULL;
> 
> This should probably become a release() method in releasing_vec.

Done.

> > +	      FOR_EACH_VEC_SAFE_ELT (*index_exp_list, i, e)
> 
> This is build_x_compound_expr_from_vec.

Done 2x.

> > +/* Wrapper for above.  */
> 
> I just applied my auto_cond_timevar patch, so you can use that instead of
> the wrapper.

Done.

> > +	  for (i = 0; i < nargs; ++i)
> > +	    {
> > +	      tree arg = CALL_EXPR_ARG (c, i);
> > +
> > +	      if (!PACK_EXPANSION_P (arg))
> > +		vec_safe_push (index_exp_list, RECUR (arg));
> > +	      else
> > +		{
> > +		  /* Expand the pack expansion and push each entry onto
> > +		     INDEX_EXP_LIST.  */
> > +		  arg = tsubst_pack_expansion (arg, args, complain, in_decl);
> > +		  if (TREE_CODE (arg) == TREE_VEC)
> > +		    {
> > +		      unsigned int len, j;
> > +
> > +		      len = TREE_VEC_LENGTH (arg);
> > +		      for (j = 0; j < len; ++j)
> > +			{
> > +			  tree value = TREE_VEC_ELT (arg, j);
> > +			  if (value != NULL_TREE)
> > +			    value = convert_from_reference (value);
> > +			  vec_safe_push (index_exp_list, value);
> > +			}
> > +		    }
> > +		  else
> > +		    {
> > +		      /* A partial substitution.  Add one entry.  */
> > +		      vec_safe_push (index_exp_list, arg);
> > +		    }
> > +		}
> > +	    }
> 
> Let's share this code with CALL_EXPR instead of duplicating it.

Done as tsubst_copy_and_build_call_args.

Tested on the new testcases so far, ok for trunk if it passes full
bootstrap/regtest?

2021-11-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102611
gcc/
	* doc/invoke.texi (-Wcomma-subscript): Document that for
	-std=c++20 the option isn't enabled by default with -Wno-deprecated
	but for -std=c++23 it is.
gcc/c-family/
	* c-opts.c (c_common_post_options): Enable -Wcomma-subscript by
	default for C++23 regardless of warn_deprecated.
	* c-cppbuiltin.c (c_cpp_builtins): Predefine
	__cpp_multidimensional_subscript=202110L for C++23.
gcc/cp/
	* cp-tree.h (build_op_subscript): Implement P2128R6
	- Multidimensional subscript operator.  Declare.
	(class releasing_vec): Add release method.
	(grok_array_decl): Remove bool argument, add vec<tree, va_gc> **
	and tsubst_flags_t arguments.
	(build_min_non_dep_op_overload): Declare another overload.
	* parser.c (cp_parser_parenthesized_expression_list_elt): New function.
	(cp_parser_postfix_open_square_expression): Mention C++23 syntax in
	function comment.  For C++23 parse zero or more than one initializer
	clauses in expression list, adjust grok_array_decl caller.
	(cp_parser_parenthesized_expression_list): Use
	cp_parser_parenthesized_expression_list_elt.
	(cp_parser_builtin_offsetof): Adjust grok_array_decl caller.
	* decl.c (grok_op_properties): For C++23 don't check number
	of arguments of operator[].
	* decl2.c (grok_array_decl): Remove decltype_p argument, add
	index_exp_list and complain arguments.  If index_exp is NULL,
	handle *index_exp_list as the subscript expression list.
	* tree.c (build_min_non_dep_op_overload): New overload.
	* call.c (add_operator_candidates, build_over_call): Adjust comments
	for removal of build_new_op_1.
	(build_op_subscript): New function.
	* pt.c (tsubst_copy_and_build_call_args): New function.
	(tsubst_copy_and_build) <case ARRAY_REF>: If second
	operand is magic CALL_EXPR with ovl_op_identifier (ARRAY_REF)
	as CALL_EXPR_FN, tsubst CALL_EXPR arguments including expanding
	pack expressions in it and call grok_array_decl instead of
	build_x_array_ref.
	<case CALL_EXPR>: Use tsubst_copy_and_build_call_args.
	* semantics.c (handle_omp_array_sections_1): Adjust grok_array_decl
	caller.
gcc/testsuite/
	* g++.dg/cpp2a/comma1.C: Expect different diagnostics for C++23.
	* g++.dg/cpp2a/comma3.C: Likewise.
	* g++.dg/cpp2a/comma4.C: Expect diagnostics for C++23.
	* g++.dg/cpp2a/comma5.C: Expect different diagnostics for C++23.
	* g++.dg/cpp23/feat-cxx2b.C: Test __cpp_multidimensional_subscript
	predefined macro.
	* g++.dg/cpp23/subscript1.C: New test.
	* g++.dg/cpp23/subscript2.C: New test.
	* g++.dg/cpp23/subscript3.C: New test.
	* g++.dg/cpp23/subscript4.C: New test.
	* g++.dg/cpp23/subscript5.C: New test.
	* g++.dg/cpp23/subscript6.C: New test.



	Jakub
  

On 11/24/21 08:37, Jakub Jelinek wrote:
> On Tue, Nov 23, 2021 at 10:28:48PM -0500, Jason Merrill wrote:
> 
> Thanks.
> 
>>> +	  while (true)
>>> +	    {
>>> +	      cp_expr expr (NULL_TREE);
>>> +	      /* Parse the next assignment-expression.  */
>>> +	      if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
>>> +		{
>>> +		  /* A braced-init-list.  */
>>> +		  bool expr_nonconst_p;
>>> +		  cp_lexer_set_source_position (parser->lexer);
>>> +		  expr = cp_parser_braced_list (parser, &expr_nonconst_p);
>>> +		}
>>> +	      else
>>> +		expr = cp_parser_assignment_expression (parser);
>>> +
>>> +	      /* If we have an ellipsis, then this is an expression
>>> +		 expansion.  */
>>> +	      if (cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
>>> +		{
>>> +		  /* Consume the `...'.  */
>>> +		  cp_lexer_consume_token (parser->lexer);
>>> +		  /* Build the argument pack.  */
>>> +		  expr = make_pack_expansion (expr);
>>> +		}
>>> +
>>> +	      if (expr == error_mark_node)
>>> +		index = error_mark_node;
>>> +	      else if (expression_list.get () == NULL
>>> +		       && !PACK_EXPANSION_P (expr.get_value ()))
>>> +		index = expr.get_value ();
>>> +	      else
>>> +		vec_safe_push (expression_list, expr.get_value ());
>>> +
>>> +	      /* If the next token isn't a `,', then we are done.  */
>>> +	      if (cp_lexer_next_token_is_not (parser->lexer, CPP_COMMA))
>>> +		break;
>>> +
>>> +	      if (expression_list.get () == NULL && index != error_mark_node)
>>> +		{
>>> +		  *&expression_list = make_tree_vector_single (index);
>>> +		  index = NULL_TREE;
>>> +		}
>>> +
>>> +	      /* Otherwise, consume the `,' and keep going.  */
>>> +	      cp_lexer_consume_token (parser->lexer);
>>> +	    }
>>
>> Let's share this loop with cp_parser_parenthesized_expression_list.
> 
> I'd prefer not to share the loop as whole because what exactly is done with
> the parsed expressions differs a lot, for the array refs I'd prefer not to
> push anything into a vector for the most common case with a single element.
> I've outlined into a function what I think I can easily share
> (see cp_parser_parenthesized_expression_list_elt in the updated patch).
> 
>>> +	  if (expression_list.get () && index == error_mark_node)
>>> +	    {
>>> +	      release_tree_vector (*&expression_list);
>>> +	      *&expression_list = NULL;
>>
>> This should probably become a release() method in releasing_vec.
> 
> Done.
> 
>>> +	      FOR_EACH_VEC_SAFE_ELT (*index_exp_list, i, e)
>>
>> This is build_x_compound_expr_from_vec.
> 
> Done 2x.
> 
>>> +/* Wrapper for above.  */
>>
>> I just applied my auto_cond_timevar patch, so you can use that instead of
>> the wrapper.
> 
> Done.
> 
>>> +	  for (i = 0; i < nargs; ++i)
>>> +	    {
>>> +	      tree arg = CALL_EXPR_ARG (c, i);
>>> +
>>> +	      if (!PACK_EXPANSION_P (arg))
>>> +		vec_safe_push (index_exp_list, RECUR (arg));
>>> +	      else
>>> +		{
>>> +		  /* Expand the pack expansion and push each entry onto
>>> +		     INDEX_EXP_LIST.  */
>>> +		  arg = tsubst_pack_expansion (arg, args, complain, in_decl);
>>> +		  if (TREE_CODE (arg) == TREE_VEC)
>>> +		    {
>>> +		      unsigned int len, j;
>>> +
>>> +		      len = TREE_VEC_LENGTH (arg);
>>> +		      for (j = 0; j < len; ++j)
>>> +			{
>>> +			  tree value = TREE_VEC_ELT (arg, j);
>>> +			  if (value != NULL_TREE)
>>> +			    value = convert_from_reference (value);
>>> +			  vec_safe_push (index_exp_list, value);
>>> +			}
>>> +		    }
>>> +		  else
>>> +		    {
>>> +		      /* A partial substitution.  Add one entry.  */
>>> +		      vec_safe_push (index_exp_list, arg);
>>> +		    }
>>> +		}
>>> +	    }
>>
>> Let's share this code with CALL_EXPR instead of duplicating it.
> 
> Done as tsubst_copy_and_build_call_args.
> 
> Tested on the new testcases so far, ok for trunk if it passes full
> bootstrap/regtest?

OK.

> 2021-11-24  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/102611
> gcc/
> 	* doc/invoke.texi (-Wcomma-subscript): Document that for
> 	-std=c++20 the option isn't enabled by default with -Wno-deprecated
> 	but for -std=c++23 it is.
> gcc/c-family/
> 	* c-opts.c (c_common_post_options): Enable -Wcomma-subscript by
> 	default for C++23 regardless of warn_deprecated.
> 	* c-cppbuiltin.c (c_cpp_builtins): Predefine
> 	__cpp_multidimensional_subscript=202110L for C++23.
> gcc/cp/
> 	* cp-tree.h (build_op_subscript): Implement P2128R6
> 	- Multidimensional subscript operator.  Declare.
> 	(class releasing_vec): Add release method.
> 	(grok_array_decl): Remove bool argument, add vec<tree, va_gc> **
> 	and tsubst_flags_t arguments.
> 	(build_min_non_dep_op_overload): Declare another overload.
> 	* parser.c (cp_parser_parenthesized_expression_list_elt): New function.
> 	(cp_parser_postfix_open_square_expression): Mention C++23 syntax in
> 	function comment.  For C++23 parse zero or more than one initializer
> 	clauses in expression list, adjust grok_array_decl caller.
> 	(cp_parser_parenthesized_expression_list): Use
> 	cp_parser_parenthesized_expression_list_elt.
> 	(cp_parser_builtin_offsetof): Adjust grok_array_decl caller.
> 	* decl.c (grok_op_properties): For C++23 don't check number
> 	of arguments of operator[].
> 	* decl2.c (grok_array_decl): Remove decltype_p argument, add
> 	index_exp_list and complain arguments.  If index_exp is NULL,
> 	handle *index_exp_list as the subscript expression list.
> 	* tree.c (build_min_non_dep_op_overload): New overload.
> 	* call.c (add_operator_candidates, build_over_call): Adjust comments
> 	for removal of build_new_op_1.
> 	(build_op_subscript): New function.
> 	* pt.c (tsubst_copy_and_build_call_args): New function.
> 	(tsubst_copy_and_build) <case ARRAY_REF>: If second
> 	operand is magic CALL_EXPR with ovl_op_identifier (ARRAY_REF)
> 	as CALL_EXPR_FN, tsubst CALL_EXPR arguments including expanding
> 	pack expressions in it and call grok_array_decl instead of
> 	build_x_array_ref.
> 	<case CALL_EXPR>: Use tsubst_copy_and_build_call_args.
> 	* semantics.c (handle_omp_array_sections_1): Adjust grok_array_decl
> 	caller.
> gcc/testsuite/
> 	* g++.dg/cpp2a/comma1.C: Expect different diagnostics for C++23.
> 	* g++.dg/cpp2a/comma3.C: Likewise.
> 	* g++.dg/cpp2a/comma4.C: Expect diagnostics for C++23.
> 	* g++.dg/cpp2a/comma5.C: Expect different diagnostics for C++23.
> 	* g++.dg/cpp23/feat-cxx2b.C: Test __cpp_multidimensional_subscript
> 	predefined macro.
> 	* g++.dg/cpp23/subscript1.C: New test.
> 	* g++.dg/cpp23/subscript2.C: New test.
> 	* g++.dg/cpp23/subscript3.C: New test.
> 	* g++.dg/cpp23/subscript4.C: New test.
> 	* g++.dg/cpp23/subscript5.C: New test.
> 	* g++.dg/cpp23/subscript6.C: New test.
> 
> --- gcc/doc/invoke.texi.jj	2021-11-24 09:54:11.537738422 +0100
> +++ gcc/doc/invoke.texi	2021-11-24 12:40:36.691189235 +0100
> @@ -3479,19 +3479,27 @@ about ABI tags.
>   @opindex Wcomma-subscript
>   @opindex Wno-comma-subscript
>   Warn about uses of a comma expression within a subscripting expression.
> -This usage was deprecated in C++20.  However, a comma expression wrapped
> -in @code{( )} is not deprecated.  Example:
> +This usage was deprecated in C++20 and is going to be removed in C++23.
> +However, a comma expression wrapped in @code{( )} is not deprecated.  Example:
>   
>   @smallexample
>   @group
>   void f(int *a, int b, int c) @{
> -    a[b,c];     // deprecated
> +    a[b,c];     // deprecated in C++20, invalid in C++23
>       a[(b,c)];   // OK
>   @}
>   @end group
>   @end smallexample
>   
> -Enabled by default with @option{-std=c++20}.
> +In C++23 it is valid to have comma separated expressions in a subscript
> +when an overloaded subscript operator is found and supports the right
> +number and types of arguments.  G++ will accept the formerly valid syntax
> +for code that is not valid in C++23 but used to be valid but deprecated
> +in C++20 with a pedantic warning that can be disabled with
> +@option{-Wno-comma-subscript}.
> +
> +Enabled by default with @option{-std=c++20} unless @option{-Wno-deprecated},
> +and with @option{-std=c++23} regardless of @option{-Wno-deprecated}.
>   
>   @item -Wctad-maybe-unsupported @r{(C++ and Objective-C++ only)}
>   @opindex Wctad-maybe-unsupported
> --- gcc/c-family/c-opts.c.jj	2021-11-16 10:01:31.314758120 +0100
> +++ gcc/c-family/c-opts.c	2021-11-24 12:40:36.692189221 +0100
> @@ -946,7 +946,8 @@ c_common_post_options (const char **pfil
>     /* -Wcomma-subscript is enabled by default in C++20.  */
>     SET_OPTION_IF_UNSET (&global_options, &global_options_set,
>   		       warn_comma_subscript,
> -		       cxx_dialect >= cxx20 && warn_deprecated);
> +		       cxx_dialect >= cxx23
> +		       || (cxx_dialect == cxx20 && warn_deprecated));
>   
>     /* -Wvolatile is enabled by default in C++20.  */
>     SET_OPTION_IF_UNSET (&global_options, &global_options_set, warn_volatile,
> --- gcc/c-family/c-cppbuiltin.c.jj	2021-11-16 10:01:31.314758120 +0100
> +++ gcc/c-family/c-cppbuiltin.c	2021-11-24 12:40:36.692189221 +0100
> @@ -1073,6 +1073,7 @@ c_cpp_builtins (cpp_reader *pfile)
>   	  cpp_define (pfile, "__cpp_size_t_suffix=202011L");
>   	  cpp_define (pfile, "__cpp_if_consteval=202106L");
>   	  cpp_define (pfile, "__cpp_constexpr=202110L");
> +	  cpp_define (pfile, "__cpp_multidimensional_subscript=202110L");
>   	}
>         if (flag_concepts)
>           {
> --- gcc/cp/cp-tree.h.jj	2021-11-24 09:54:11.457739563 +0100
> +++ gcc/cp/cp-tree.h	2021-11-24 13:06:41.083861782 +0100
> @@ -1007,7 +1007,9 @@ public:
>        (bootstrap/91828).  */
>     tree& operator[] (ptrdiff_t i) const { return (*v)[i]; }
>   
> -  ~releasing_vec() { release_tree_vector (v); }
> +  void release () { release_tree_vector (v); v = NULL; }
> +
> +  ~releasing_vec () { release_tree_vector (v); }
>   private:
>     vec_t *v;
>   };
> @@ -6471,6 +6473,9 @@ inline tree build_new_op (const op_locat
>   }
>   extern tree build_op_call			(tree, vec<tree, va_gc> **,
>   						 tsubst_flags_t);
> +extern tree build_op_subscript			(const op_location_t &, tree,
> +						 vec<tree, va_gc> **, tree *,
> +						 tsubst_flags_t);
>   extern bool aligned_allocation_fn_p		(tree);
>   extern tree destroying_delete_p			(tree);
>   extern bool usual_deallocation_fn_p		(tree);
> @@ -6813,7 +6818,8 @@ extern void maybe_make_one_only			(tree)
>   extern bool vague_linkage_p			(tree);
>   extern void grokclassfn				(tree, tree,
>   						 enum overload_flags);
> -extern tree grok_array_decl			(location_t, tree, tree, bool);
> +extern tree grok_array_decl			(location_t, tree, tree,
> +						 vec<tree, va_gc> **, tsubst_flags_t);
>   extern tree delete_sanity			(location_t, tree, tree, bool,
>   						 int, tsubst_flags_t);
>   extern tree check_classfn			(tree, tree, tree);
> @@ -7711,6 +7717,8 @@ extern tree build_min_nt_loc			(location
>   						 ...);
>   extern tree build_min_non_dep			(enum tree_code, tree, ...);
>   extern tree build_min_non_dep_op_overload	(enum tree_code, tree, tree, ...);
> +extern tree build_min_non_dep_op_overload	(tree, tree, tree,
> +						 vec<tree, va_gc> *);
>   extern tree build_min_nt_call_vec (tree, vec<tree, va_gc> *);
>   extern tree build_min_non_dep_call_vec		(tree, tree, vec<tree, va_gc> *);
>   extern vec<tree, va_gc>* vec_copy_and_insert    (vec<tree, va_gc>*, tree, unsigned);
> --- gcc/cp/parser.c.jj	2021-11-24 09:54:11.497738993 +0100
> +++ gcc/cp/parser.c	2021-11-24 14:11:28.599152722 +0100
> @@ -7898,11 +7898,62 @@ cp_parser_postfix_expression (cp_parser
>     return error_mark_node;
>   }
>   
> +/* Helper function for cp_parser_parenthesized_expression_list and
> +   cp_parser_postfix_open_square_expression.  Parse a single element
> +   of parenthesized expression list.  */
> +
> +static cp_expr
> +cp_parser_parenthesized_expression_list_elt (cp_parser *parser, bool cast_p,
> +					     bool allow_expansion_p,
> +					     bool fold_expr_p,
> +					     bool *non_constant_p)
> +{
> +  cp_expr expr (NULL_TREE);
> +  bool expr_non_constant_p;
> +
> +  /* Parse the next assignment-expression.  */
> +  if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
> +    {
> +      /* A braced-init-list.  */
> +      cp_lexer_set_source_position (parser->lexer);
> +      maybe_warn_cpp0x (CPP0X_INITIALIZER_LISTS);
> +      expr = cp_parser_braced_list (parser, &expr_non_constant_p);
> +      if (non_constant_p && expr_non_constant_p)
> +	*non_constant_p = true;
> +    }
> +  else if (non_constant_p)
> +    {
> +      expr = cp_parser_constant_expression (parser,
> +					    /*allow_non_constant_p=*/true,
> +					    &expr_non_constant_p);
> +      if (expr_non_constant_p)
> +	*non_constant_p = true;
> +    }
> +  else
> +    expr = cp_parser_assignment_expression (parser, /*pidk=*/NULL, cast_p);
> +
> +  if (fold_expr_p)
> +    expr = instantiate_non_dependent_expr (expr);
> +
> +  /* If we have an ellipsis, then this is an expression expansion.  */
> +  if (allow_expansion_p
> +      && cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
> +    {
> +      /* Consume the `...'.  */
> +      cp_lexer_consume_token (parser->lexer);
> +
> +      /* Build the argument pack.  */
> +      expr = make_pack_expansion (expr);
> +    }
> +  return expr;
> +}
> +
>   /* A subroutine of cp_parser_postfix_expression that also gets hijacked
>      by cp_parser_builtin_offsetof.  We're looking for
>   
>        postfix-expression [ expression ]
>        postfix-expression [ braced-init-list ] (C++11)
> +     postfix-expression [ expression-list[opt] ] (C++23)
>   
>      FOR_OFFSETOF is set if we're being called in that context, which
>      changes how we deal with integer constant expressions.  */
> @@ -7914,6 +7965,7 @@ cp_parser_postfix_open_square_expression
>   					  bool decltype_p)
>   {
>     tree index = NULL_TREE;
> +  releasing_vec expression_list = NULL;
>     location_t loc = cp_lexer_peek_token (parser->lexer)->location;
>     bool saved_greater_than_is_operator_p;
>   
> @@ -7935,7 +7987,49 @@ cp_parser_postfix_open_square_expression
>       index = cp_parser_constant_expression (parser);
>     else
>       {
> -      if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
> +      if (cxx_dialect >= cxx23
> +	  && cp_lexer_next_token_is (parser->lexer, CPP_CLOSE_SQUARE))
> +	*&expression_list = make_tree_vector ();
> +      else if (cxx_dialect >= cxx23)
> +	{
> +	  while (true)
> +	    {
> +	      cp_expr expr
> +		= cp_parser_parenthesized_expression_list_elt (parser,
> +							       /*cast_p=*/
> +							       false,
> +							       /*allow_exp_p=*/
> +							       true,
> +							       /*fold_expr_p=*/
> +							       false,
> +							       /*non_cst_p=*/
> +							       NULL);
> +
> +	      if (expr == error_mark_node)
> +		index = error_mark_node;
> +	      else if (expression_list.get () == NULL
> +		       && !PACK_EXPANSION_P (expr.get_value ()))
> +		index = expr.get_value ();
> +	      else
> +		vec_safe_push (expression_list, expr.get_value ());
> +
> +	      /* If the next token isn't a `,', then we are done.  */
> +	      if (cp_lexer_next_token_is_not (parser->lexer, CPP_COMMA))
> +		break;
> +
> +	      if (expression_list.get () == NULL && index != error_mark_node)
> +		{
> +		  *&expression_list = make_tree_vector_single (index);
> +		  index = NULL_TREE;
> +		}
> +
> +	      /* Otherwise, consume the `,' and keep going.  */
> +	      cp_lexer_consume_token (parser->lexer);
> +	    }
> +	  if (expression_list.get () && index == error_mark_node)
> +	    expression_list.release ();
> +	}
> +      else if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
>   	{
>   	  bool expr_nonconst_p;
>   	  cp_lexer_set_source_position (parser->lexer);
> @@ -7955,7 +8049,9 @@ cp_parser_postfix_open_square_expression
>   
>     /* Build the ARRAY_REF.  */
>     postfix_expression = grok_array_decl (loc, postfix_expression,
> -					index, decltype_p);
> +					index, &expression_list,
> +					tf_warning_or_error
> +					| (decltype_p ? tf_decltype : 0));
>   
>     /* When not doing offsetof, array references are not permitted in
>        constant-expressions.  */
> @@ -8315,44 +8411,11 @@ cp_parser_parenthesized_expression_list
>   	  }
>   	else
>   	  {
> -	    bool expr_non_constant_p;
> -
> -	    /* Parse the next assignment-expression.  */
> -	    if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
> -	      {
> -		/* A braced-init-list.  */
> -		cp_lexer_set_source_position (parser->lexer);
> -		maybe_warn_cpp0x (CPP0X_INITIALIZER_LISTS);
> -		expr = cp_parser_braced_list (parser, &expr_non_constant_p);
> -		if (non_constant_p && expr_non_constant_p)
> -		  *non_constant_p = true;
> -	      }
> -	    else if (non_constant_p)
> -	      {
> -		expr = (cp_parser_constant_expression
> -			(parser, /*allow_non_constant_p=*/true,
> -			 &expr_non_constant_p));
> -		if (expr_non_constant_p)
> -		  *non_constant_p = true;
> -	      }
> -	    else
> -	      expr = cp_parser_assignment_expression (parser, /*pidk=*/NULL,
> -						      cast_p);
> -
> -	    if (fold_expr_p)
> -	      expr = instantiate_non_dependent_expr (expr);
> -
> -            /* If we have an ellipsis, then this is an expression
> -	       expansion.  */
> -            if (allow_expansion_p
> -                && cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
> -              {
> -                /* Consume the `...'.  */
> -                cp_lexer_consume_token (parser->lexer);
> -
> -                /* Build the argument pack.  */
> -                expr = make_pack_expansion (expr);
> -              }
> +	    expr
> +	      = cp_parser_parenthesized_expression_list_elt (parser, cast_p,
> +							     allow_expansion_p,
> +							     fold_expr_p,
> +							     non_constant_p);
>   
>   	    if (wrap_locations_p)
>   	      expr.maybe_add_location_wrapper ();
> @@ -10625,8 +10688,8 @@ cp_parser_builtin_offsetof (cp_parser *p
>   
>   	case CPP_DEREF:
>   	  /* offsetof-member-designator "->" identifier */
> -	  expr = grok_array_decl (token->location, expr,
> -				  integer_zero_node, false);
> +	  expr = grok_array_decl (token->location, expr, integer_zero_node,
> +				  NULL, tf_warning_or_error);
>   	  /* FALLTHRU */
>   
>   	case CPP_DOT:
> --- gcc/cp/decl.c.jj	2021-11-24 09:54:11.474739321 +0100
> +++ gcc/cp/decl.c	2021-11-24 12:40:36.699189121 +0100
> @@ -15140,6 +15140,8 @@ grok_op_properties (tree decl, bool comp
>       case OVL_OP_FLAG_BINARY:
>         if (arity != 2)
>   	{
> +	  if (operator_code == ARRAY_REF && cxx_dialect >= cxx23)
> +	    break;
>   	  error_at (loc,
>   		    methodp
>   		    ? G_("%qD must have exactly one argument")
> --- gcc/cp/decl2.c.jj	2021-11-05 00:43:22.511625568 +0100
> +++ gcc/cp/decl2.c	2021-11-24 13:15:41.845142933 +0100
> @@ -363,16 +363,20 @@ grokclassfn (tree ctype, tree function,
>   }
>   
>   /* Create an ARRAY_REF, checking for the user doing things backwards
> -   along the way.  DECLTYPE_P is for N3276, as in the parser.  */
> +   along the way.
> +   If INDEX_EXP is non-NULL, then that is the index expression,
> +   otherwise INDEX_EXP_LIST is the list of index expressions.  */
>   
>   tree
>   grok_array_decl (location_t loc, tree array_expr, tree index_exp,
> -		 bool decltype_p)
> +		 vec<tree, va_gc> **index_exp_list, tsubst_flags_t complain)
>   {
>     tree type;
>     tree expr;
>     tree orig_array_expr = array_expr;
>     tree orig_index_exp = index_exp;
> +  vec<tree, va_gc> *orig_index_exp_list
> +    = index_exp_list ? *index_exp_list : NULL;
>     tree overload = NULL_TREE;
>   
>     if (error_operand_p (array_expr) || error_operand_p (index_exp))
> @@ -381,11 +385,23 @@ grok_array_decl (location_t loc, tree ar
>     if (processing_template_decl)
>       {
>         if (type_dependent_expression_p (array_expr)
> -	  || type_dependent_expression_p (index_exp))
> -	return build_min_nt_loc (loc, ARRAY_REF, array_expr, index_exp,
> -				 NULL_TREE, NULL_TREE);
> +	  || (index_exp ? type_dependent_expression_p (index_exp)
> +			: any_type_dependent_arguments_p (*index_exp_list)))
> +	{
> +	  if (index_exp == NULL)
> +	    index_exp = build_min_nt_call_vec (ovl_op_identifier (ARRAY_REF),
> +					       *index_exp_list);
> +	  return build_min_nt_loc (loc, ARRAY_REF, array_expr, index_exp,
> +				   NULL_TREE, NULL_TREE);
> +	}
>         array_expr = build_non_dependent_expr (array_expr);
> -      index_exp = build_non_dependent_expr (index_exp);
> +      if (index_exp)
> +	index_exp = build_non_dependent_expr (index_exp);
> +      else
> +	{
> +	  orig_index_exp_list = make_tree_vector_copy (*index_exp_list);
> +	  make_args_non_dependent (*index_exp_list);
> +	}
>       }
>   
>     type = TREE_TYPE (array_expr);
> @@ -393,13 +409,44 @@ grok_array_decl (location_t loc, tree ar
>     type = non_reference (type);
>   
>     /* If they have an `operator[]', use that.  */
> -  if (MAYBE_CLASS_TYPE_P (type) || MAYBE_CLASS_TYPE_P (TREE_TYPE (index_exp)))
> +  if (MAYBE_CLASS_TYPE_P (type)
> +      || (index_exp && MAYBE_CLASS_TYPE_P (TREE_TYPE (index_exp)))
> +      || (index_exp == NULL_TREE
> +	  && !(*index_exp_list)->is_empty ()
> +	  && MAYBE_CLASS_TYPE_P (TREE_TYPE ((*index_exp_list)->last ()))))
>       {
> -      tsubst_flags_t complain = tf_warning_or_error;
> -      if (decltype_p)
> -	complain |= tf_decltype;
> -      expr = build_new_op (loc, ARRAY_REF, LOOKUP_NORMAL, array_expr,
> -			   index_exp, NULL_TREE, &overload, complain);
> +      if (index_exp)
> +	expr = build_new_op (loc, ARRAY_REF, LOOKUP_NORMAL, array_expr,
> +			     index_exp, NULL_TREE, &overload, complain);
> +      else if ((*index_exp_list)->is_empty ())
> +	expr = build_op_subscript (loc, array_expr, index_exp_list, &overload,
> +				   complain);
> +      else
> +	{
> +	  expr = build_op_subscript (loc, array_expr, index_exp_list,
> +				     &overload, complain & tf_decltype);
> +	  if (expr == error_mark_node)
> +	    {
> +	      tree idx = build_x_compound_expr_from_vec (*index_exp_list, NULL,
> +							 tf_none);
> +	      if (idx != error_mark_node)
> +		expr = build_new_op (loc, ARRAY_REF, LOOKUP_NORMAL, array_expr,
> +				     idx, NULL_TREE, &overload,
> +				     complain & tf_decltype);
> +	      if (expr == error_mark_node)
> +		{
> +		  overload = NULL_TREE;
> +		  expr = build_op_subscript (loc, array_expr, index_exp_list,
> +					     &overload, complain);
> +		}
> +	      else
> +		/* If it would be valid albeit deprecated expression in C++20,
> +		   just pedwarn on it and treat it as if wrapped in ().  */
> +		pedwarn (loc, OPT_Wcomma_subscript,
> +			 "top-level comma expression in array subscript "
> +			 "changed meaning in C++23");
> +	    }
> +	}
>       }
>     else
>       {
> @@ -415,6 +462,31 @@ grok_array_decl (location_t loc, tree ar
>         else
>   	p1 = build_expr_type_conversion (WANT_POINTER, array_expr, false);
>   
> +      if (index_exp == NULL_TREE)
> +	{
> +	  if ((*index_exp_list)->is_empty ())
> +	    {
> +	      error_at (loc, "built-in subscript operator without expression "
> +			     "list");
> +	      return error_mark_node;
> +	    }
> +	  tree idx = build_x_compound_expr_from_vec (*index_exp_list, NULL,
> +						     tf_none);
> +	  if (idx != error_mark_node)
> +	    /* If it would be valid albeit deprecated expression in C++20,
> +	       just pedwarn on it and treat it as if wrapped in ().  */
> +	    pedwarn (loc, OPT_Wcomma_subscript,
> +		     "top-level comma expression in array subscript "
> +		     "changed meaning in C++23");
> +	  else
> +	    {
> +	      error_at (loc, "built-in subscript operator with more than one "
> +			     "expression in expression list");
> +	      return error_mark_node;
> +	    }
> +	  index_exp = idx;
> +	}
> +
>         if (TREE_CODE (TREE_TYPE (index_exp)) == ARRAY_TYPE)
>   	p2 = index_exp;
>         else
> @@ -457,11 +529,30 @@ grok_array_decl (location_t loc, tree ar
>     if (processing_template_decl && expr != error_mark_node)
>       {
>         if (overload != NULL_TREE)
> -	return (build_min_non_dep_op_overload
> -		(ARRAY_REF, expr, overload, orig_array_expr, orig_index_exp));
> +	{
> +	  if (orig_index_exp == NULL_TREE)
> +	    {
> +	      expr = build_min_non_dep_op_overload (expr, overload,
> +						    orig_array_expr,
> +						    orig_index_exp_list);
> +	      release_tree_vector (orig_index_exp_list);
> +	      return expr;
> +	    }
> +	  return build_min_non_dep_op_overload (ARRAY_REF, expr, overload,
> +						orig_array_expr,
> +						orig_index_exp);
> +	}
> +
> +      if (orig_index_exp == NULL_TREE)
> +	{
> +	  orig_index_exp
> +	    = build_min_nt_call_vec (ovl_op_identifier (ARRAY_REF),
> +				     orig_index_exp_list);
> +	  release_tree_vector (orig_index_exp_list);
> +	}
>   
> -      return build_min_non_dep (ARRAY_REF, expr, orig_array_expr, orig_index_exp,
> -				NULL_TREE, NULL_TREE);
> +      return build_min_non_dep (ARRAY_REF, expr, orig_array_expr,
> +				orig_index_exp, NULL_TREE, NULL_TREE);
>       }
>     return expr;
>   }
> --- gcc/cp/tree.c.jj	2021-11-19 16:39:51.534595887 +0100
> +++ gcc/cp/tree.c	2021-11-24 12:40:36.700189107 +0100
> @@ -3671,13 +3671,42 @@ build_min_non_dep_op_overload (enum tree
>   	}
>       }
>     else
> -   gcc_unreachable ();
> +    gcc_unreachable ();
>   
>     va_end (p);
>     call = build_min_non_dep_call_vec (non_dep, fn, args);
>   
>     tree call_expr = extract_call_expr (call);
>     KOENIG_LOOKUP_P (call_expr) = KOENIG_LOOKUP_P (non_dep);
> +  CALL_EXPR_OPERATOR_SYNTAX (call_expr) = true;
> +  CALL_EXPR_ORDERED_ARGS (call_expr) = CALL_EXPR_ORDERED_ARGS (non_dep);
> +  CALL_EXPR_REVERSE_ARGS (call_expr) = CALL_EXPR_REVERSE_ARGS (non_dep);
> +
> +  return call;
> +}
> +
> +/* Similar to above build_min_non_dep_op_overload, but arguments
> +   are taken from ARGS vector.  */
> +
> +tree
> +build_min_non_dep_op_overload (tree non_dep, tree overload, tree object,
> +			       vec<tree, va_gc> *args)
> +{
> +  non_dep = extract_call_expr (non_dep);
> +
> +  unsigned int nargs = call_expr_nargs (non_dep);
> +  gcc_assert (TREE_CODE (TREE_TYPE (overload)) == METHOD_TYPE);
> +  tree binfo = TYPE_BINFO (TREE_TYPE (object));
> +  tree method = build_baselink (binfo, binfo, overload, NULL_TREE);
> +  tree fn = build_min (COMPONENT_REF, TREE_TYPE (overload),
> +		       object, method, NULL_TREE);
> +  nargs--;
> +  gcc_assert (vec_safe_length (args) == nargs);
> +
> +  tree call = build_min_non_dep_call_vec (non_dep, fn, args);
> +
> +  tree call_expr = extract_call_expr (call);
> +  KOENIG_LOOKUP_P (call_expr) = KOENIG_LOOKUP_P (non_dep);
>     CALL_EXPR_OPERATOR_SYNTAX (call_expr) = true;
>     CALL_EXPR_ORDERED_ARGS (call_expr) = CALL_EXPR_ORDERED_ARGS (non_dep);
>     CALL_EXPR_REVERSE_ARGS (call_expr) = CALL_EXPR_REVERSE_ARGS (non_dep);
> --- gcc/cp/call.c.jj	2021-11-24 09:54:11.456739577 +0100
> +++ gcc/cp/call.c	2021-11-24 14:11:02.262530064 +0100
> @@ -6283,7 +6283,7 @@ op_is_ordered (tree_code code)
>       }
>   }
>   
> -/* Subroutine of build_new_op_1: Add to CANDIDATES all candidates for the
> +/* Subroutine of build_new_op: Add to CANDIDATES all candidates for the
>      operator indicated by CODE/CODE2.  This function calls itself recursively to
>      handle C++20 rewritten comparison operator candidates.  */
>   
> @@ -6932,6 +6932,117 @@ build_new_op (const op_location_t &loc,
>     return NULL_TREE;
>   }
>   
> +/* Build a new call to operator[].  This may change ARGS.  */
> +
> +tree
> +build_op_subscript (const op_location_t &loc, tree obj,
> +		    vec<tree, va_gc> **args, tree *overload,
> +		    tsubst_flags_t complain)
> +{
> +  struct z_candidate *candidates = 0, *cand;
> +  tree fns, first_mem_arg = NULL_TREE;
> +  bool any_viable_p;
> +  tree result = NULL_TREE;
> +  void *p;
> +
> +  auto_cond_timevar tv (TV_OVERLOAD);
> +
> +  obj = mark_lvalue_use (obj);
> +
> +  if (error_operand_p (obj))
> +    return error_mark_node;
> +
> +  tree type = TREE_TYPE (obj);
> +
> +  obj = prep_operand (obj);
> +
> +  if (TYPE_BINFO (type))
> +    {
> +      fns = lookup_fnfields (TYPE_BINFO (type), ovl_op_identifier (ARRAY_REF),
> +			     1, complain);
> +      if (fns == error_mark_node)
> +	return error_mark_node;
> +    }
> +  else
> +    fns = NULL_TREE;
> +
> +  if (args != NULL && *args != NULL)
> +    {
> +      *args = resolve_args (*args, complain);
> +      if (*args == NULL)
> +	return error_mark_node;
> +    }
> +
> +  /* Get the high-water mark for the CONVERSION_OBSTACK.  */
> +  p = conversion_obstack_alloc (0);
> +
> +  if (fns)
> +    {
> +      first_mem_arg = obj;
> +
> +      add_candidates (BASELINK_FUNCTIONS (fns),
> +		      first_mem_arg, *args, NULL_TREE,
> +		      NULL_TREE, false,
> +		      BASELINK_BINFO (fns), BASELINK_ACCESS_BINFO (fns),
> +		      LOOKUP_NORMAL, &candidates, complain);
> +    }
> +
> +  /* Be strict here because if we choose a bad conversion candidate, the
> +     errors we get won't mention the call context.  */
> +  candidates = splice_viable (candidates, true, &any_viable_p);
> +  if (!any_viable_p)
> +    {
> +      if (complain & tf_error)
> +	{
> +	  auto_diagnostic_group d;
> +	  error ("no match for call to %<%T::operator[] (%A)%>",
> +		 TREE_TYPE (obj), build_tree_list_vec (*args));
> +	  print_z_candidates (loc, candidates);
> +	}
> +      result = error_mark_node;
> +    }
> +  else
> +    {
> +      cand = tourney (candidates, complain);
> +      if (cand == 0)
> +	{
> +	  if (complain & tf_error)
> +	    {
> +	      auto_diagnostic_group d;
> +	      error ("call of %<%T::operator[] (%A)%> is ambiguous",
> +		     TREE_TYPE (obj), build_tree_list_vec (*args));
> +	      print_z_candidates (loc, candidates);
> +	    }
> +	  result = error_mark_node;
> +	}
> +      else if (TREE_CODE (cand->fn) == FUNCTION_DECL
> +	       && DECL_OVERLOADED_OPERATOR_P (cand->fn)
> +	       && DECL_OVERLOADED_OPERATOR_IS (cand->fn, ARRAY_REF))
> +	{
> +	  if (overload)
> +	    *overload = cand->fn;
> +	  result = build_over_call (cand, LOOKUP_NORMAL, complain);
> +	  if (trivial_fn_p (cand->fn) || DECL_IMMEDIATE_FUNCTION_P (cand->fn))
> +	    /* There won't be a CALL_EXPR.  */;
> +	  else if (result && result != error_mark_node)
> +	    {
> +	      tree call = extract_call_expr (result);
> +	      CALL_EXPR_OPERATOR_SYNTAX (call) = true;
> +
> +	      /* Specify evaluation order as per P0145R2.  */
> +	      CALL_EXPR_ORDERED_ARGS (call) = op_is_ordered (ARRAY_REF) == 1;
> +	    }
> +	}
> +      else
> +	gcc_unreachable ();
> +    }
> +
> +  /* Free all the conversions we allocated.  */
> +  obstack_free (&conversion_obstack, p);
> +
> +  return result;
> +}
> +
>   /* CALL was returned by some call-building function; extract the actual
>      CALL_EXPR from any bits that have been tacked on, e.g. by
>      convert_from_reference.  */
> @@ -9748,7 +9859,7 @@ build_over_call (struct z_candidate *can
>     if (cand->flags & LOOKUP_LIST_INIT_CTOR)
>       {
>         tree c = extract_call_expr (call);
> -      /* build_new_op_1 will clear this when appropriate.  */
> +      /* build_new_op will clear this when appropriate.  */
>         CALL_EXPR_ORDERED_ARGS (c) = true;
>       }
>     if (warned_p)
> --- gcc/cp/pt.c.jj	2021-11-23 15:09:15.633870147 +0100
> +++ gcc/cp/pt.c	2021-11-24 13:49:45.607858720 +0100
> @@ -19654,6 +19654,49 @@ maybe_fold_fn_template_args (tree fn, ts
>     return fold_targs_r (targs, complain);
>   }
>   
> +/* Helper function for tsubst_copy_and_build CALL_EXPR and ARRAY_REF
> +   handling.  */
> +
> +static void
> +tsubst_copy_and_build_call_args (tree t, tree args, tsubst_flags_t complain,
> +				 tree in_decl,
> +				 bool integral_constant_expression_p,
> +				 releasing_vec &call_args)
> +{
> +  unsigned int nargs = call_expr_nargs (t);
> +  for (unsigned int i = 0; i < nargs; ++i)
> +    {
> +      tree arg = CALL_EXPR_ARG (t, i);
> +
> +      if (!PACK_EXPANSION_P (arg))
> +	vec_safe_push (call_args,
> +		       tsubst_copy_and_build (arg, args, complain, in_decl,
> +					      /*function_p=*/false,
> +					      integral_constant_expression_p));
> +      else
> +	{
> +	  /* Expand the pack expansion and push each entry onto CALL_ARGS.  */
> +	  arg = tsubst_pack_expansion (arg, args, complain, in_decl);
> +	  if (TREE_CODE (arg) == TREE_VEC)
> +	    {
> +	      unsigned int len, j;
> +
> +	      len = TREE_VEC_LENGTH (arg);
> +	      for (j = 0; j < len; ++j)
> +		{
> +		  tree value = TREE_VEC_ELT (arg, j);
> +		  if (value != NULL_TREE)
> +		    value = convert_from_reference (value);
> +		  vec_safe_push (call_args, value);
> +		}
> +	    }
> +	  else
> +	    /* A partial substitution.  Add one entry.  */
> +	    vec_safe_push (call_args, arg);
> +	}
> +    }
> +}
> +
>   /* Like tsubst but deals with expressions and performs semantic
>      analysis.  FUNCTION_P is true if T is the "F" in "F (ARGS)" or
>      "F<TARGS> (ARGS)".  */
> @@ -20053,6 +20096,28 @@ tsubst_copy_and_build (tree t,
>       case ARRAY_REF:
>         op1 = tsubst_non_call_postfix_expression (TREE_OPERAND (t, 0),
>   						args, complain, in_decl);
> +      if (TREE_CODE (TREE_OPERAND (t, 1)) == CALL_EXPR
> +	  && (CALL_EXPR_FN (TREE_OPERAND (t, 1))
> +	      == ovl_op_identifier (ARRAY_REF)))
> +	{
> +	  tree c = TREE_OPERAND (t, 1);
> +	  releasing_vec index_exp_list;
> +	  tsubst_copy_and_build_call_args (c, args, complain, in_decl,
> +					   integral_constant_expression_p,
> +					   index_exp_list);
> +
> +	  tree r;
> +	  if (vec_safe_length (index_exp_list) == 1
> +	      && !PACK_EXPANSION_P (index_exp_list[0]))
> +	    r = grok_array_decl (EXPR_LOCATION (t), op1,
> +				 index_exp_list[0], NULL,
> +				 complain | decltype_flag);
> +	  else
> +	    r = grok_array_decl (EXPR_LOCATION (t), op1,
> +				 NULL_TREE, &index_exp_list,
> +				 complain | decltype_flag);
> +	  RETURN (r);
> +	}
>         RETURN (build_x_array_ref (EXPR_LOCATION (t), op1,
>   				 RECUR (TREE_OPERAND (t, 1)),
>   				 complain|decltype_flag));
> @@ -20261,7 +20326,7 @@ tsubst_copy_and_build (tree t,
>       case CALL_EXPR:
>         {
>   	tree function;
> -	unsigned int nargs, i;
> +	unsigned int nargs;
>   	bool qualified_p;
>   	bool koenig_p;
>   	tree ret;
> @@ -20344,37 +20409,9 @@ tsubst_copy_and_build (tree t,
>   
>   	nargs = call_expr_nargs (t);
>   	releasing_vec call_args;
> -	for (i = 0; i < nargs; ++i)
> -	  {
> -	    tree arg = CALL_EXPR_ARG (t, i);
> -
> -	    if (!PACK_EXPANSION_P (arg))
> -	      vec_safe_push (call_args, RECUR (CALL_EXPR_ARG (t, i)));
> -	    else
> -	      {
> -		/* Expand the pack expansion and push each entry onto
> -		   CALL_ARGS.  */
> -		arg = tsubst_pack_expansion (arg, args, complain, in_decl);
> -		if (TREE_CODE (arg) == TREE_VEC)
> -		  {
> -		    unsigned int len, j;
> -
> -		    len = TREE_VEC_LENGTH (arg);
> -		    for (j = 0; j < len; ++j)
> -		      {
> -			tree value = TREE_VEC_ELT (arg, j);
> -			if (value != NULL_TREE)
> -			  value = convert_from_reference (value);
> -			vec_safe_push (call_args, value);
> -		      }
> -		  }
> -		else
> -		  {
> -		    /* A partial substitution.  Add one entry.  */
> -		    vec_safe_push (call_args, arg);
> -		  }
> -	      }
> -	  }
> +	tsubst_copy_and_build_call_args (t, args, complain, in_decl,
> +					 integral_constant_expression_p,
> +					 call_args);
>   
>   	/* Stripped-down processing for a call in a thunk.  Specifically, in
>   	   the thunk template for a generic lambda.  */
> --- gcc/cp/semantics.c.jj	2021-11-23 11:03:43.988325187 +0100
> +++ gcc/cp/semantics.c	2021-11-24 12:40:36.705189036 +0100
> @@ -5394,7 +5394,8 @@ handle_omp_array_sections_1 (tree c, tre
>   		      OMP_CLAUSE_CODE (c) == OMP_CLAUSE_REDUCTION
>   		      || OMP_CLAUSE_CODE (c) == OMP_CLAUSE_IN_REDUCTION
>   		      || OMP_CLAUSE_CODE (c) == OMP_CLAUSE_TASK_REDUCTION);
> -  ret = grok_array_decl (OMP_CLAUSE_LOCATION (c), ret, low_bound, false);
> +  ret = grok_array_decl (OMP_CLAUSE_LOCATION (c), ret, low_bound, NULL,
> +			 tf_warning_or_error);
>     return ret;
>   }
>   
> --- gcc/testsuite/g++.dg/cpp2a/comma1.C.jj	2021-10-15 11:58:45.210130057 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/comma1.C	2021-11-24 12:40:37.107183299 +0100
> @@ -8,19 +8,24 @@ struct S {
>   void
>   fn (int *a, int b, int c)
>   {
> -  a[b,c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20 } }
> +  a[b,c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_only } }
> +	  // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(b,c)];
>   
> -  a[(void) b, c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20 } }
> +  a[(void) b, c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_only } }
> +		  // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[((void) b, c)];
>   
> -  a[(void) b, (void) c, (void) b, b]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20 } }
> +  a[(void) b, (void) c, (void) b, b]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_only } }
> +				      // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[((void) b, (void) c, (void) b, b)];
>   
> -  a[S(), 10]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20 } }
> +  a[S(), 10]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_only } }
> +	      // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(S(), 10)];
>   
>     a[int{(1,2)}];
> -  a[int{(1,2)}, int{}]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20 } }
> +  a[int{(1,2)}, int{}]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_only } }
> +			// { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(int{(1,2)}, int{})];
>   }
> --- gcc/testsuite/g++.dg/cpp2a/comma3.C.jj	2021-10-15 11:58:45.210130057 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/comma3.C	2021-11-24 12:40:37.119183128 +0100
> @@ -9,19 +9,24 @@ struct S {
>   void
>   fn (int *a, int b, int c)
>   {
> -  a[b,c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[b,c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +	  // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(b,c)];
>   
> -  a[(void) b, c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[(void) b, c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +		  // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[((void) b, c)];
>   
> -  a[(void) b, (void) c, (void) b, b]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[(void) b, (void) c, (void) b, b]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +				      // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[((void) b, (void) c, (void) b, b)];
>   
> -  a[S(), 10]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[S(), 10]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +	      // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(S(), 10)];
>   
>     a[int{(1,2)}];
> -  a[int{(1,2)}, int{}]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[int{(1,2)}, int{}]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +			// { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(int{(1,2)}, int{})];
>   }
> --- gcc/testsuite/g++.dg/cpp2a/comma4.C.jj	2021-10-15 11:58:45.210130057 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/comma4.C	2021-11-24 12:40:37.122183085 +0100
> @@ -10,18 +10,23 @@ void
>   fn (int *a, int b, int c)
>   {
>     a[b,c]; // { dg-bogus "top-level comma expression in array subscript is deprecated" }
> +	  // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(b,c)];
>   
>     a[(void) b, c]; // { dg-bogus "top-level comma expression in array subscript is deprecated" }
> +		  // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[((void) b, c)];
>   
>     a[(void) b, (void) c, (void) b, b]; // { dg-bogus "top-level comma expression in array subscript is deprecated" }
> +				      // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[((void) b, (void) c, (void) b, b)];
>   
>     a[S(), 10]; // { dg-bogus "top-level comma expression in array subscript is deprecated" }
> +	      // { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(S(), 10)];
>   
>     a[int{(1,2)}];
>     a[int{(1,2)}, int{}]; // { dg-bogus "top-level comma expression in array subscript is deprecated" }
> +			// { dg-warning "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(int{(1,2)}, int{})];
>   }
> --- gcc/testsuite/g++.dg/cpp2a/comma5.C.jj	2021-10-15 11:58:45.210130057 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/comma5.C	2021-11-24 12:40:37.126183028 +0100
> @@ -8,14 +8,20 @@ void
>   fn (int *a, int b, int c)
>   {
>     a[foo<int, int>(1, 2)];
> -  a[foo<int, int>(1, 2), foo<int, int>(3, 4)]; // { dg-warning "24:top-level comma expression in array subscript is deprecated" }
> +  a[foo<int, int>(1, 2), foo<int, int>(3, 4)]; // { dg-warning "24:top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +					       // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>   
> -  a[b < c, b < c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> -  a[b < c, b > c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> -  a[b > c, b > c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> -  a[b > c, b < c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[b < c, b < c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +		   // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
> +  a[b < c, b > c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +		   // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
> +  a[b > c, b > c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +		   // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
> +  a[b > c, b < c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +		   // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(b < c, b < c)];
>     a[(b < c, b > c)];
> -  a[b << c, b << c]; // { dg-warning "top-level comma expression in array subscript is deprecated" }
> +  a[b << c, b << c]; // { dg-warning "top-level comma expression in array subscript is deprecated" "" { target c++20_down } }
> +		     // { dg-error "top-level comma expression in array subscript changed meaning in" "" { target c++23 } .-1 }
>     a[(b << c, b << c)];
>   }
> --- gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C.jj	2021-10-15 11:58:45.192130317 +0200
> +++ gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C	2021-11-24 12:40:37.131182956 +0100
> @@ -551,3 +551,9 @@
>   #elif __cpp_if_consteval != 202106
>   #  error "__cpp_if_consteval != 202106"
>   #endif
> +
> +#ifndef __cpp_multidimensional_subscript
> +#  error "__cpp_multidimensional_subscript"
> +#elif __cpp_multidimensional_subscript != 202110
> +#  error "__cpp_multidimensional_subscript != 202110"
> +#endif
> --- gcc/testsuite/g++.dg/cpp23/subscript1.C.jj	2021-11-24 12:40:37.132182942 +0100
> +++ gcc/testsuite/g++.dg/cpp23/subscript1.C	2021-11-24 12:40:37.132182942 +0100
> @@ -0,0 +1,55 @@
> +// P2128R6
> +// { dg-do run }
> +// { dg-options "-std=c++23" }
> +
> +extern "C" void abort ();
> +
> +struct S
> +{
> +  constexpr S () : a {} {};
> +  constexpr S (int x, int y, int z) : a {x, y, z} {};
> +  constexpr int &operator[] () { return a[0]; }
> +  constexpr int &operator[] (int x) { return a[x]; }
> +  constexpr int &operator[] (int x, long y) { return a[x + y * 8]; }
> +  int a[64];
> +};
> +
> +struct T
> +{
> +  operator int () { return 42; };
> +};
> +
> +int buf[64];
> +
> +struct U
> +{
> +  operator int * () { return buf; }
> +};
> +
> +static_assert (S ()[1] == 0);
> +static_assert (S (1, 2, 42)[2] == 42);
> +static_assert (S ()[3, 4] == 0);
> +static_assert (S (1, 43, 2)[1, 0] == 43);
> +static_assert (S ()[] == 0);
> +static_assert (S (44, 1, 2)[] == 44);
> +
> +int
> +main ()
> +{
> +  S s;
> +  for (int i = 0; i < 64; i++)
> +    s.a[i] = 64 - i;
> +  if (s[] != 64 || s[3] != 61 || s[4, 5] != 20)
> +    abort ();
> +  s[]++;
> +  s[42]++;
> +  ++s[3, 2];
> +  if (s.a[0] != 65 || s.a[42] != 23 || s.a[19] != 46)
> +    abort ();
> +  T t;
> +  U u;
> +  if (&u[t] != &buf[42])
> +    abort ();
> +  if (&t[u] != &buf[42])
> +    abort ();
> +}
> --- gcc/testsuite/g++.dg/cpp23/subscript2.C.jj	2021-11-24 12:40:37.132182942 +0100
> +++ gcc/testsuite/g++.dg/cpp23/subscript2.C	2021-11-24 12:40:37.132182942 +0100
> @@ -0,0 +1,51 @@
> +// P2128R6
> +// { dg-do compile }
> +// { dg-options "-std=c++23" }
> +
> +struct S
> +{
> +  S () : a {} {};
> +  int &operator[] () { return a[0]; }
> +  int &operator[] (int x) { return a[x]; }
> +  int &operator[] (int x, long y) { return a[x + y * 8]; }
> +  int a[64];
> +};
> +
> +struct T
> +{
> +  operator int () { return 42; };
> +};
> +
> +int buf[64];
> +
> +struct U
> +{
> +  operator int * () { return buf; }
> +};
> +
> +struct V
> +{
> +  V () : a {} {};
> +  V (int x, int y, int z) : a {x, y, z} {};
> +  int &operator[] () { return a[0]; }				// { dg-message "candidate" }
> +  int &operator[] (int x, long y) { return a[x + y * 8]; }	// { dg-message "candidate" }
> +  int a[64];
> +};
> +
> +void
> +foo ()
> +{
> +  S s;
> +  T t;
> +  U u;
> +  V v;
> +  auto &a = buf[];		// { dg-error "built-in subscript operator without expression list" }
> +  auto &b = buf[1, 2];		// { dg-warning "top-level comma expression in array subscript changed meaning in" }
> +  auto &c = s[1, 2, 3];		// { dg-warning "top-level comma expression in array subscript changed meaning in" }
> +  auto &d = v[1];		// { dg-error "no match for 'operator\\\[\\\]' in 'v\\\[1\\\]' \\\(operand types are 'V' and 'int'\\\)" }
> +  auto &e = v[1, 2, 3];		// { dg-error "no match for call to 'V::operator\\\[\\\] \\\(int, int, int\\\)'" }
> +  auto &f = t[42, u];		// { dg-warning "top-level comma expression in array subscript changed meaning in" }
> +  auto &g = u[42, t];		// { dg-warning "top-level comma expression in array subscript changed meaning in" }
> +  auto &h = buf[42, 2.5];	// { dg-warning "top-level comma expression in array subscript changed meaning in" }
> +				// { dg-error "invalid types \[^\n\r]* for array subscript" "" { target *-*-* } .-1 }
> +}
> --- gcc/testsuite/g++.dg/cpp23/subscript3.C.jj	2021-11-24 12:40:37.132182942 +0100
> +++ gcc/testsuite/g++.dg/cpp23/subscript3.C	2021-11-24 12:40:37.132182942 +0100
> @@ -0,0 +1,90 @@
> +// P2128R6
> +// { dg-do run }
> +// { dg-options "-std=c++23" }
> +
> +extern "C" void abort ();
> +
> +struct S
> +{
> +  constexpr S () : a {} {};
> +  constexpr S (int x, int y, int z) : a {x, y, z} {};
> +  constexpr int &operator[] () { return a[0]; }
> +  constexpr int &operator[] (int x) { return a[x]; }
> +  constexpr int &operator[] (int x, long y) { return a[x + y * 8]; }
> +  int a[64];
> +};
> +
> +struct T
> +{
> +  operator int () { return 42; };
> +};
> +
> +int buf[64];
> +
> +struct U
> +{
> +  operator int * () { return buf; }
> +};
> +
> +template <int N>
> +void
> +foo ()
> +{
> +  static_assert (S ()[1] == 0);
> +  static_assert (S (1, 2, 42)[2] == 42);
> +  static_assert (S ()[3, 4] == 0);
> +  static_assert (S (1, 43, 2)[1, 0] == 43);
> +  static_assert (S ()[] == 0);
> +  static_assert (S (44, 1, 2)[] == 44);
> +  S s;
> +  for (int i = 0; i < 64; i++)
> +    s.a[i] = 64 - i;
> +  if (s[] != 64 || s[3] != 61 || s[4, 5] != 20)
> +    abort ();
> +  s[]++;
> +  s[42]++;
> +  ++s[3, 2];
> +  if (s.a[0] != 65 || s.a[42] != 23 || s.a[19] != 46)
> +    abort ();
> +  T t;
> +  U u;
> +  if (&u[t] != &buf[42])
> +    abort ();
> +  if (&t[u] != &buf[42])
> +    abort ();
> +}
> +
> +template <typename V, typename W, typename X>
> +void
> +bar ()
> +{
> +  static_assert (V ()[1] == 0);
> +  static_assert (V (1, 2, 42)[2] == 42);
> +  static_assert (V ()[3, 4] == 0);
> +  static_assert (V (1, 43, 2)[1, 0] == 43);
> +  static_assert (V ()[] == 0);
> +  static_assert (V (44, 1, 2)[] == 44);
> +  V s;
> +  for (int i = 0; i < 64; i++)
> +    s.a[i] = 64 - i;
> +  if (s[] != 64 || s[3] != 61 || s[4, 5] != 20)
> +    abort ();
> +  s[]++;
> +  s[42]++;
> +  ++s[3, 2];
> +  if (s.a[0] != 65 || s.a[42] != 23 || s.a[19] != 46)
> +    abort ();
> +  W t;
> +  X u;
> +  if (&u[t] != &buf[42])
> +    abort ();
> +  if (&t[u] != &buf[42])
> +    abort ();
> +}
> +
> +int
> +main ()
> +{
> +  foo <0> ();
> +  bar <S, T, U> ();
> +}
> --- gcc/testsuite/g++.dg/cpp23/subscript4.C.jj	2021-11-24 12:40:37.132182942 +0100
> +++ gcc/testsuite/g++.dg/cpp23/subscript4.C	2021-11-24 12:40:37.132182942 +0100
> @@ -0,0 +1,44 @@
> +// P2128R6
> +// { dg-do run }
> +// { dg-options "-std=c++23" }
> +
> +extern "C" void abort ();
> +
> +struct S
> +{
> +  constexpr S () : a {} {};
> +  constexpr S (int x, int y, int z) : a {x, y, z} {};
> +  constexpr int &operator[] () { return a[0]; }
> +  constexpr int &operator[] (int x) { return a[x]; }
> +  constexpr int &operator[] (int x, long y) { return a[x + y * 8]; }
> +  int a[64];
> +};
> +int buf[26];
> +
> +template <class ...Ts>
> +auto &
> +foo (S &s, Ts... args)
> +{
> +  return s[args...];
> +}
> +
> +template <typename T, class ...Ts>
> +auto &
> +bar (T &s, Ts... args)
> +{
> +  return s[args...];
> +}
> +
> +int
> +main ()
> +{
> +  S s;
> +  if (&foo (s) != &s.a[0]
> +      || &foo (s, 42) != &s.a[42]
> +      || &foo (s, 5, 4) != &s.a[37]
> +      || &bar (s) != &s.a[0]
> +      || &bar (s, 22) != &s.a[22]
> +      || &bar (s, 17, 3L) != &s.a[41]
> +      || &bar (buf, 5) != &buf[5])
> +    abort ();
> +}
> --- gcc/testsuite/g++.dg/cpp23/subscript5.C.jj	2021-11-24 12:40:37.132182942 +0100
> +++ gcc/testsuite/g++.dg/cpp23/subscript5.C	2021-11-24 12:40:37.132182942 +0100
> @@ -0,0 +1,28 @@
> +// P2128R6
> +// { dg-do run { target c++11 } }
> +
> +#include <initializer_list>
> +#include <cstdlib>
> +
> +struct S
> +{
> +  S () : a {} {};
> +  int &operator[] (std::initializer_list<int> l) {
> +    int sum = 0;
> +    for (auto x : l)
> +      sum += x;
> +    return a[sum];
> +  }
> +  int a[64];
> +};
> +
> +int
> +main ()
> +{
> +  S s;
> +  if (&s[{0}] != &s.a[0]
> +      || &s[{42}] != &s.a[42]
> +      || &s[{5, 7, 9}] != &s.a[5 + 7 + 9]
> +      || &s[{1, 2, 3, 4}] != &s.a[1 + 2 + 3 + 4])
> +    abort ();
> +}
> --- gcc/testsuite/g++.dg/cpp23/subscript6.C.jj	2021-11-24 12:40:37.132182942 +0100
> +++ gcc/testsuite/g++.dg/cpp23/subscript6.C	2021-11-24 12:40:37.132182942 +0100
> @@ -0,0 +1,31 @@
> +// P2128R6
> +// { dg-do run }
> +// { dg-options "-std=c++23" }
> +
> +#include <initializer_list>
> +#include <cstdlib>
> +
> +struct S
> +{
> +  S () : a {} {};
> +  int &operator[] (std::initializer_list<int> l, std::initializer_list<int> m) {
> +    int sum = 0;
> +    for (auto x : l)
> +      sum += x;
> +    for (auto x : m)
> +      sum += x;
> +    return a[sum];
> +  }
> +  int a[64];
> +};
> +
> +int
> +main ()
> +{
> +  S s;
> +  if (&s[{0}, {3, 1, 2}] != &s.a[0 + 3 + 1 + 2]
> +      || &s[{42}, {11, 1}] != &s.a[42 + 11 + 1]
> +      || &s[{5, 7, 9}, {3}] != &s.a[5 + 7 + 9 + 3]
> +      || &s[{1, 2, 3, 4}, {3, 5, 8}] != &s.a[1 + 2 + 3 + 4 + 3 + 5 + 8])
> +    abort ();
> +}
> 
> 
> 	Jakub
>