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

[Jason Merrill <jason@redhat.com>]

On 10/28/21 07:26, Jakub Jelinek wrote:
> On Wed, Oct 27, 2021 at 04:58:53PM -0400, Jason Merrill wrote:
>> On 10/21/21 04:42, Jakub Jelinek wrote:
>>> Hi!
>>>
>>> Here is an attempt to implement DR2351 - void{} - where void{} after
>>> pack expansion is considered valid and the same thing as void().
>>> For templates, dunno if we have some better way to check if a CONSTRUCTOR
>>> might be empty after pack expansion.  Would that only if the constructor
>>> only contains EXPR_PACK_EXPANSION elements and nothing else, or something
>>> else too?
>>
>> I think that's the only case.  For template args there's the
>> pack_expansion_args_count function, but I don't think there's anything
>> similar for constructor elts; please feel free to add it.
> 
> Ok.  But counting how many packs its CONSTRUCTOR_ELTS have and then comparing
> that number against CONSTRUCTOR_NELTS seems to be unnecessarily expensive if
> there are many elements, for the purpose the DR2351 code needs we can stop
> as soon as we see first non-pack element.
> 
> So what about this if it passes bootstrap/regtest?
> 
> 2021-10-28  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/102820
> 	* semantics.c (maybe_zero_constructor_nelts): New function.
> 	(finish_compound_literal): Implement DR2351 - void{}.
> 	If type is cv void and compound_literal has no elements, return
> 	void_node.  If type is cv void and compound_literal might have no
> 	elements after expansion, handle it like other dependent compound
> 	literals.
> 
> 	* g++.dg/cpp0x/dr2351.C: New test.
> 
> --- 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))

> +    if (!PACK_EXPANSION_P (val))
> +      return false;
> +  return true;
> +}
> +
>   /* Finish a compound-literal expression or C++11 functional cast with aggregate
>      initializer.  TYPE is the type to which the CONSTRUCTOR in COMPOUND_LITERAL
>      is being cast.  */
> @@ -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).

OK with those tweaks.

> +      else if (VOID_TYPE_P (type)
> +	       && processing_template_decl
> +	       && maybe_zero_constructor_nelts (compound_literal))
> +	/* If there are only packs in compound_literal, it could
> +	   be void{} after pack expansion.  */;
> +      else
> +	{
> +	  if (complain & tf_error)
> +	    error ("compound literal of non-object type %qT", type);
> +	  return error_mark_node;
> +	}
>       }
>   
>     if (template_placeholder_p (type))
> --- gcc/testsuite/g++.dg/cpp0x/dr2351.C.jj	2021-10-28 12:59:27.987120315 +0200
> +++ gcc/testsuite/g++.dg/cpp0x/dr2351.C	2021-10-28 13:15:20.532760871 +0200
> @@ -0,0 +1,51 @@
> +// DR2351
> +// { dg-do compile { target c++11 } }
> +
> +void
> +foo ()
> +{
> +  void{};
> +  void();
> +}
> +
> +template <class ...T>
> +void
> +bar (T... t)
> +{
> +  void{t...};
> +  void(t...);
> +}
> +
> +void
> +baz ()
> +{
> +  bar ();
> +}
> +
> +template <class ...T>
> +void
> +qux (T... t)
> +{
> +  void{t...};	// { dg-error "compound literal of non-object type" }
> +}
> +
> +void
> +corge ()
> +{
> +  qux (1, 2);
> +}
> +
> +template <class ...T>
> +void
> +garply (T... t)
> +{
> +  void{t..., t..., t...};
> +  void(t..., t..., t...);
> +}
> +
> +template <class ...T>
> +void
> +grault (T... t)
> +{
> +  void{t..., 1};	// { dg-error "compound literal of non-object type" }
> +}
> 
> 
> 	Jakub
>