c++: Implement DR2351 - void{} [PR102820]

  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?  With the patch as is we wouldn't diagnose
template <class ...T>
void
bar (T... t)
{
  void{1, t...};
}
at parsing time, only at instantiation time, even when it will always
expand to at least one CONSTRUCTOR elt.

Bootstrapped/regtested on x86_64-linux and i686-linux.

2021-10-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102820
	* semantics.c (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 is instantiation
	dependent, handle it like other dependent compound literals.

	* g++.dg/cpp0x/dr2351.C: New test.


	Jakub
  

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.

> With the patch as is we wouldn't diagnose
> template <class ...T>
> void
> bar (T... t)
> {
>    void{1, t...};
> }
> at parsing time, only at instantiation time, even when it will always
> expand to at least one CONSTRUCTOR elt.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux.
> 
> 2021-10-21  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/102820
> 	* semantics.c (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 is instantiation
> 	dependent, handle it like other dependent compound literals.
> 
> 	* g++.dg/cpp0x/dr2351.C: New test.
> 
> --- gcc/cp/semantics.c.jj	2021-10-15 11:58:45.079131947 +0200
> +++ gcc/cp/semantics.c	2021-10-20 17:00:38.586705600 +0200
> @@ -3104,9 +3104,20 @@ 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;
> +      else if (VOID_TYPE_P (type)
> +	       && processing_template_decl
> +	       && instantiation_dependent_expression_p (compound_literal))
> +	/* If there are 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	2021-10-20 17:06:02.399162937 +0200
> +++ gcc/testsuite/g++.dg/cpp0x/dr2351.C	2021-10-20 17:05:54.294276511 +0200
> @@ -0,0 +1,36 @@
> +// 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);
> +}
> 
> 	Jakub
>