Re: [PATCH] c++: Check for indirect change of active union member in constexpr [PR101631]

[Jason Merrill <jason@redhat.com>]

On 8/29/23 09:35, Nathaniel Shead wrote:
> This is an attempt to improve the constexpr machinery's handling of
> union lifetime by catching more cases that cause UB. Is this approach
> OK?
> 
> I'd also like some feedback on a couple of pain points with this
> implementation; in particular, is there a good way to detect if a type
> has a non-deleted trivial constructor? I've used 'is_trivially_xible' in
> this patch, but that also checks for a trivial destructor which by my
> reading of [class.union.general]p5 is possibly incorrect. Checking for a
> trivial default constructor doesn't seem too hard but I couldn't find a
> good way of checking if that constructor is deleted.

I guess the simplest would be

(TYPE_HAS_TRIVIAL_DFLT (t) && locate_ctor (t))

because locate_ctor returns null for a deleted default ctor.  It would 
be good to make this a separate predicate.

> I'm also generally unsatisfied with the additional complexity with the
> third 'refs' argument in 'cxx_eval_store_expression' being pushed and
> popped; would it be better to replace this with a vector of some
> specific structure type for the data that needs to be passed on?

Perhaps, but what you have here is fine.  Another possibility would be 
to just have a vec of the refs and extract the index from the ref later 
as needed.

Jason