[PATCH] c++: missing SFINAE in grok_array_decl [PR111493]

[Jason Merrill]

On 9/20/23 11:03, Patrick Palka wrote:
> On Wed, 20 Sep 2023, Patrick Palka wrote:
> 
>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>> trunk?
>>
>> -- >8 --
>>
>> This fixes some missed SFINAE in grok_array_decl when checking a C++23
>> multidimensional subscript operator expression.
>>
>> Note the existing pedwarn code paths are a backward compability fallback
>> for treating invalid a[x, y, z] as a[(x, y, z)], but this should only be
>> done outside of a SFINAE context I think.
>>
>> 	PR c++/111493
>>
>> gcc/cp/ChangeLog:
>>
>> 	* decl2.cc (grok_array_decl): Guard errors with tf_error.
>> 	In the pedwarn code paths, return error_mark_node when in
>> 	a SFINAE context.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	* g++.dg/cpp23/subscript15.C: New test.
>> ---
>>   gcc/cp/decl2.cc                          | 36 +++++++++++++++---------
>>   gcc/testsuite/g++.dg/cpp23/subscript15.C | 24 ++++++++++++++++
>>   2 files changed, 47 insertions(+), 13 deletions(-)
>>   create mode 100644 gcc/testsuite/g++.dg/cpp23/subscript15.C
>>
>> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
>> index b402befba6d..6eb6d8c57d6 100644
>> --- a/gcc/cp/decl2.cc
>> +++ b/gcc/cp/decl2.cc
>> @@ -477,7 +477,9 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
>>   		{
>>   		  /* If it would be valid albeit deprecated expression in
>>   		     C++20, just pedwarn on it and treat it as if wrapped
>> -		     in ().  */
>> +		     in () unless we're in a SFINAE context.  */
>> +		  if (!(complain & tf_error))
>> +		    return error_mark_node;
> 
> It occurred to me that we could check for tf_error much earlier, before
> we call build_x_compound_expr_from_vec and build_new_op, since they're
> only used here to implement the backward compatibilty fallback.  Perhaps
> the following is better, then:

This version is OK.

> -- >8 --
> 
> Subject: [PATCH] c++: missing SFINAE in grok_array_decl [PR111493]
> 
> We should guard both the diagnostic and backward compatibilty fallback
> code with tf_error, so that in a SFINAE context we don't issue any
> diagnostics and correctly recognize ill-formed C++23 multidimensional
> subscript operator expressions.
> 
> 	PR c++/111493
> 
> gcc/cp/ChangeLog:
> 
> 	* decl2.cc (grok_array_decl): Guard diagnostic and backward
> 	compatibility fallback code paths with tf_error.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp23/subscript15.C: New test.
> ---
>   gcc/cp/decl2.cc                          | 15 +++++++++++---
>   gcc/testsuite/g++.dg/cpp23/subscript15.C | 25 ++++++++++++++++++++++++
>   2 files changed, 37 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp23/subscript15.C
> 
> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
> index b402befba6d..6ac27cbc15f 100644
> --- a/gcc/cp/decl2.cc
> +++ b/gcc/cp/decl2.cc
> @@ -459,7 +459,10 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
>   	{
>   	  expr = build_op_subscript (loc, array_expr, index_exp_list,
>   				     &overload, complain & tf_decltype);
> -	  if (expr == error_mark_node)
> +	  if (expr == error_mark_node
> +	      /* Don't do the backward compatibility fallback in a SFINAE
> +		 context.   */
> +	      && (complain & tf_error))
>   	    {
>   	      tree idx = build_x_compound_expr_from_vec (*index_exp_list, NULL,
>   							 tf_none);
> @@ -510,6 +513,11 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
>   
>         if (index_exp == NULL_TREE)
>   	{
> +	  if (!(complain & tf_error))
> +	    /* Don't do the backward compatibility fallback in a SFINAE
> +	       context.  */
> +	    return error_mark_node;
> +
>   	  if ((*index_exp_list)->is_empty ())
>   	    {
>   	      error_at (loc, "built-in subscript operator without expression "
> @@ -561,8 +569,9 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
>   	swapped = true, array_expr = p2, index_exp = i1;
>         else
>   	{
> -	  error_at (loc, "invalid types %<%T[%T]%> for array subscript",
> -		    type, TREE_TYPE (index_exp));
> +	  if (complain & tf_error)
> +	    error_at (loc, "invalid types %<%T[%T]%> for array subscript",
> +		      type, TREE_TYPE (index_exp));
>   	  return error_mark_node;
>   	}
>   
> diff --git a/gcc/testsuite/g++.dg/cpp23/subscript15.C b/gcc/testsuite/g++.dg/cpp23/subscript15.C
> new file mode 100644
> index 00000000000..fece96be96b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/subscript15.C
> @@ -0,0 +1,25 @@
> +// PR c++/111493
> +// { dg-do compile { target c++23 } }
> +
> +template<class T, class... Ts>
> +concept CartesianIndexable = requires(T t, Ts... ts) { t[ts...]; };
> +
> +static_assert(!CartesianIndexable<int>);
> +static_assert(!CartesianIndexable<int, int>);
> +static_assert(!CartesianIndexable<int, int, int>);
> +
> +static_assert(!CartesianIndexable<int*>);
> +static_assert(CartesianIndexable<int*, int>);
> +static_assert(!CartesianIndexable<int*, int, int>);
> +static_assert(!CartesianIndexable<int*, int*>);
> +
> +template<class... Ts>
> +struct A {
> +  void operator[](Ts...);
> +};
> +
> +static_assert(!CartesianIndexable<A<>, int>);
> +static_assert(CartesianIndexable<A<int>, int>);
> +static_assert(!CartesianIndexable<A<int>>);
> +static_assert(!CartesianIndexable<A<int>, int, int>);
> +static_assert(CartesianIndexable<A<int, int>, int, int>);