[PATCH] c++, libstdc++, v5: Implement C++26 P3726R2 - Adjustments to Union Lifetime Rules [PR125845]

[Marek Polacek]

On Wed, Sep 09, 2026 at 05:22:24PM +0200, Jakub Jelinek wrote:
> On Wed, Sep 09, 2026 at 10:00:48AM -0400, Jason Merrill wrote:
> > On 9/9/26 6:08 AM, Jakub Jelinek wrote:
> > > On Wed, Sep 09, 2026 at 12:06:14PM +0200, Tomasz Kaminski wrote:
> --- a/gcc/cp/cp-gimplify.cc	2026-09-09 17:03:29.375511108 +0200
> +++ b/gcc/cp/cp-gimplify.cc	2026-09-09 17:08:09.020290280 +0200
> @@ -1040,6 +1040,7 @@ cp_gimplify_expr (tree *expr_p, gimple_s
>  								    0));
>  		break;
>  	      case CP_BUILT_IN_CONSTEXPR_DIAG:
> +	      case CP_BUILT_IN_START_LIFETIME:
>  		*expr_p = void_node;

Is it safe to drop this when the argument has side-effects?

>  		break;
>  	      default:
> --- a/gcc/cp/semantics.cc	2026-09-09 17:03:29.386510966 +0200
> +++ b/gcc/cp/semantics.cc	2026-09-09 17:08:09.021167493 +0200
> @@ -3300,6 +3300,82 @@ perform_koenig_lookup (cp_expr fn_expr,
>    return cp_expr (fn, loc);
>  }
>  
> +/* Analyze call to a front-end builtin FN with arguments *ARGS.  Return true
> +   if the call is correct, false if erroneous.  */
> +
> +static bool
> +check_frontend_builtin (tree fn, vec<tree, va_gc> **args,
> +			tsubst_flags_t complain)
> +{
> +  tree arg, ptype, type;
> +  switch (DECL_UNCHECKED_FUNCTION_CODE (fn))
> +    {
> +    case CP_BUILT_IN_IS_WITHIN_LIFETIME:
> +      /* Unless users call the builtin directly, the following 2 checks
> +	 should be ensured from std::is_within_lifetime template.  */
> +      if (vec_safe_length (*args) != 1)
> +	{
> +	  if (complain & tf_error)
> +	    error ("%qs needs a single argument",
> +		   "__builtin_is_within_lifetime");
> +	  return false;
> +	}
> +      arg = (**args)[0];
> +      if (error_operand_p (arg))
> +	return false;
> +      ptype = TREE_TYPE (arg);
> +      if (!POINTER_TYPE_P (ptype))
> +	{
> +	  if (complain & tf_error)
> +	    error ("%qs argument type %qT is not pointer type",
> +		   "__builtin_is_within_lifetime", ptype);
> +	  return false;
> +	}
> +      return true;
> +    case CP_BUILT_IN_START_LIFETIME:

This:

> +      /* Unless users call the builtin directly, the following 2 checks
> +	 should be ensured from std::start_lifetime template.  */
> +      if (vec_safe_length (*args) != 1)
> +	{
> +	  if (complain & tf_error)
> +	    error ("%qs needs a single argument", "__builtin_start_lifetime");
> +	  return false;
> +	}
> +      arg = (**args)[0];
> +      if (error_operand_p (arg))
> +	return false;
> +      ptype = TREE_TYPE (arg);
> +      if (!POINTER_TYPE_P (ptype))
> +	{
> +	  if (complain & tf_error)
> +	    error ("%qs argument type %qT is not pointer type",
> +		   "__builtin_start_lifetime", ptype);
> +	  return false;
> +	}

seems to duplicate the previous case almost verbatim.

> +      type = TREE_TYPE (ptype);
> +      if (!complete_type_or_else (type, NULL_TREE))

This emits a diagnostic unconditionally, should it be
complete_type_or_maybe_complain?  Elsewhere this function
checks complain first.

Marek