Re: [C++ PATCH] Fix __builtin_{is_constant_evaluated,constant_p} handling in static_assert (PR c++/86524, PR c++/88446)

[Jason Merrill <jason@redhat.com>]

On 12/20/18 4:40 PM, Jakub Jelinek wrote:
> On Thu, Dec 20, 2018 at 04:26:21PM -0500, Jason Merrill wrote:
>> Right, but they either are or they aren't.  Doing this isn't likely to help
>> anything, and can lead to paradoxical results in contrived testcases.
>>
>>> But if you think this is a bad idea, I can remove it, I haven't been
>>> successful in constructing a testcase where this would matter.
>>
>> Please.  OK with those changes.
> 
> Actually, the main reason for any changes in that block was mainly that I thought
> it is a bad idea to fold the argument with __builtin_is_const_evaluated ()
> folded into true if that argument isn't a constant expression.
> 
> So are you ok with what is in the patch below, i.e.
>         {
>           bool non_cst_p = false, ovf_p = false;
>           tree a = cxx_eval_constant_expression (&new_ctx, args[i], false,
>                                                  &non_cst_p, &ovf_p);
>           if ((!non_cst_p && !ovf_p) || !ctx->manifestly_const_eval)
>             args[i] = a;
>         }
> , or perhaps without the || !ctx->manifestly_const_eval?

I don't see how that makes a difference from what was there before; if 
the argument to cxx_eval_constant_expression is non-constant, it returns 
the argument unchanged.

> So, if the
> argument is a constant expression, fold to that, if it is not, just do
> cp_fully_fold on it if it is __builtin_constant_p, otherwise nothing?

Hmm, cp_fully_fold probably also needs to add a manifestly_const_eval 
parameter to pass along to maybe_constant_value.

Jason