Re: [PATCH] c++, v2: Implement DR2351 - void{} [PR102820]

[Jason Merrill <jason@redhat.com>]

On 10/28/21 08:19, Jakub Jelinek wrote:
> On Thu, Oct 28, 2021 at 08:01:27AM -0400, Jason Merrill wrote:
>>> --- gcc/cp/semantics.c.jj	2021-10-27 09:16:41.161600606 +0200
>>> +++ gcc/cp/semantics.c	2021-10-28 13:06:59.325791588 +0200
>>> @@ -3079,6 +3079,24 @@ finish_unary_op_expr (location_t op_loc,
>>>      return result;
>>>    }
>>> +/* Return true if CONSTRUCTOR EXPR after pack expansion could have no
>>> +   elements.  */
>>> +
>>> +static bool
>>> +maybe_zero_constructor_nelts (tree expr)
>>> +{
>>> +  if (CONSTRUCTOR_NELTS (expr) == 0)
>>> +    return true;
>>> +  if (!processing_template_decl)
>>> +    return false;
>>> +  unsigned int i;
>>> +  tree val;
>>> +  FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (expr), i, val)
>>
>> Let's use
>>
>>        for (constructor_elt &elt : CONSTRUCTOR_ELTS (t))
> 
> Ok, will do.
> 
>>> @@ -3104,9 +3122,20 @@ finish_compound_literal (tree type, tree
>>>      if (!TYPE_OBJ_P (type))
>>>        {
>>> -      if (complain & tf_error)
>>> -	error ("compound literal of non-object type %qT", type);
>>> -      return error_mark_node;
>>> +      /* DR2351 */
>>> +      if (VOID_TYPE_P (type) && CONSTRUCTOR_NELTS (compound_literal) == 0)
>>> +	return void_node;
>>
>> This test now seems redundant with the one below (if you remove the &&
>> processing_template_decl).
> 
> It is not redundant, for the maybe case it doesn't return void_node, but
> falls through into if (processing_template_decl), which, because
> compound_literal is necessarily instantiation_dependent_expression_p
> (it contains packs) will just create CONSTRUCTOR_IS_DEPENDENT CONSTRUCTOR
> and we'll get here back during instantiation.
> For the CONSTRUCTOR_NELTS == 0 case even in templates we know
> compound_literal isn't dependent (it doesn't contain anything) and type
> isn't either, so we can return void_node right away (and when
> !processing_template_decl we have to do that).

Ah, right.  Never mind that comment, then.

Jason