Re: [PATCH] c++: Fix up mangling ICE with void{} [PR106863]

[Jason Merrill <jason@redhat.com>]

On 10/19/22 04:00, Jakub Jelinek wrote:
> Hi!
> 
> We ICE on the following testcase during mangling, finish_compound_literal
> returns for void{} void_node and the mangler doesn't handle it.
> Handling void_node in the mangler seems problematic to me, because
> we don't know for which case it has been created.
> The following patch arranges to mangle just void{} the same as void()
> if that is what we want to use, by doing what we do for void() when
> processing void{}.
> The code does that only if processing_template_decl, because otherwise
> build_functional_cast will return void_node, so calling it looks like
> wasted effort to me.  But if you want to call it unconditionally,
> I can certainly do that too.

I think in a template we want the same early-return behavior as in the 
processing_template_decl block farther down in the function: 
specifically, we want to return a CONSTRUCTOR (for which 
COMPOUND_LITERAL_P is true), so it mangles as void{} rather than void().

> Or do you want to mangle it differently?  How?
> 
> clang++ doesn't support DR2351, so I can't check what they are doing.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux.
> 
> 2022-10-19  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/106863
> 	* semantics.cc (finish_compound_literal): For void{}, if
> 	processing_template_decl return build_functional_cast of NULL_TREE
> 	to VOID_TYPE rather than void_node.
> 
> 	* g++.dg/cpp0x/dr2351-2.C: New test.
> 
> --- gcc/cp/semantics.cc.jj	2022-10-10 09:31:57.410985121 +0200
> +++ gcc/cp/semantics.cc	2022-10-18 15:24:08.726026118 +0200
> @@ -3164,7 +3164,12 @@ finish_compound_literal (tree type, tree
>       {
>         /* DR2351 */
>         if (VOID_TYPE_P (type) && CONSTRUCTOR_NELTS (compound_literal) == 0)
> -	return void_node;
> +	{
> +	  if (!processing_template_decl)
> +	    return void_node;
> +	  location_t loc = cp_expr_loc_or_input_loc (compound_literal);
> +	  return build_functional_cast (loc, type, NULL_TREE, complain);
> +	}
>         else if (VOID_TYPE_P (type)
>   	       && processing_template_decl
>   	       && maybe_zero_constructor_nelts (compound_literal))
> --- gcc/testsuite/g++.dg/cpp0x/dr2351-2.C.jj	2022-10-18 15:27:01.146690132 +0200
> +++ gcc/testsuite/g++.dg/cpp0x/dr2351-2.C	2022-10-18 15:27:39.909164970 +0200
> @@ -0,0 +1,16 @@
> +// DR2351
> +// { dg-do compile { target c++11 } }
> +
> +void bar (int);
> +
> +template <typename T>
> +auto foo (T t) -> decltype (bar (t), void{})
> +{
> +  return bar (t);
> +}
> +
> +int
> +main ()
> +{
> +  foo (0);
> +}
> 
> 	Jakub
>