Re: [PATCH v4 1/3] c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675]

[Jason Merrill <jason@redhat.com>]

On 7/20/23 05:35, Nathaniel Shead wrote:
> This adds rudimentary lifetime tracking in C++ constexpr contexts,
> allowing the compiler to report errors with using values after their
> backing has gone out of scope. We don't yet handle other ways of
> accessing values outside their lifetime (e.g. following explicit
> destructor calls).

Incidentally, much of that should be straightforward to handle by no 
longer ignoring clobbers here:

>     case MODIFY_EXPR:
>       if (cxx_dialect < cxx14)
>         goto fail;
>       if (!RECUR (TREE_OPERAND (t, 0), any))
>         return false;
>       /* Just ignore clobbers.  */
>       if (TREE_CLOBBER_P (TREE_OPERAND (t, 1)))
>         return true;

Assignment from a clobber represents end of lifetime to the middle-end. 
This can be a follow-up patch.

> @@ -7051,10 +7065,17 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
>   	return ctx->ctor;
>         if (VAR_P (t))
>   	if (tree v = ctx->global->get_value (t))
> -	    {
> -	      r = v;
> -	      break;
> -	    }
> +	  {
> +	    r = v;
> +	    break;
> +	  }
> +      if (ctx->global->is_outside_lifetime (t))
> +	{
> +	  if (!ctx->quiet)
> +	    outside_lifetime_error (loc, t);
> +	  *non_constant_p = true;
> +	  break;
> +	}

Shouldn't this new check also be under the if (VAR_P (t))?  A CONST_DECL 
can't go out of scope.

Jason