[PATCH] c++, libstdc++, v4: Implement C++26 P2641R4 - Checking if a union alternative is active

[Jason Merrill]

On 9/1/26 11:53 AM, Jakub Jelinek wrote:
> +  if (!valp || *valp == void_list_node)
> +    {
> +      if (!ctx->quiet)
> +	{
> +	  auto_diagnostic_group d;
> +	  if (DECL_P (obj) && DECL_NAME (obj) == heap_deleted_identifier)
> +	    {
> +	      error_at (loc, "%qs on allocated storage after deallocation "
> +			"is not a constant expression",
> +			"__builtin_is_within_lifetime");
> +	      inform (DECL_SOURCE_LOCATION (obj), "allocated here");
> +	    }
> +	  else if (DECL_P (obj) && ctx->global->is_outside_lifetime (obj))
> +	    {
> +	      error_at (loc, "%qs on %qE outside its lifetime is not a "
> +			"constant expression",

Let's also clarify this message to talk about storage.  OK with that 
adjustment.

Jason