Re: [PATCH v3 1/3] c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675]

[Jason Merrill <jason@redhat.com>]

On 7/16/23 09:47, Nathaniel Shead wrote:
> On Fri, Jul 14, 2023 at 11:16:58AM -0400, Jason Merrill wrote:
>>
>> What if, instead of removing the variable from one hash table and adding it
>> to another, we change the value to, say, void_node?
> 
> I have another patch I'm working on after this which does seem to
> require the overlapping tables to properly catch uses of aggregates
> while they are still being constructed (i.e. before their lifetime has
> begun), as part of PR c++/109518. In that case the 'values' map contains
> the CONSTRUCTOR node for the aggregate, but it also needs to be in
> 'outside_lifetime'. I could also explore solving this another way
> however if you prefer.

I'd think to handle this with a flag on the CONSTRUCTOR to indicate that 
members with no value are out of lifetime (so, a stronger version of 
CONSTRUCTOR_NO_CLEARING that just indicates uninitialized).  Currently 
all the TREE_LANG_FLAG_* are occupied on CONSTRUCTOR, but there seem to 
be plenty of spare bits to add a TREE_LANG_FLAG_7.

It might make sense to access those two flags with accessor functions so 
they stay aligned.

> (I also have vague dreams of at some point making this a map to the
> location that the object was destroyed for more context in the error
> messages, but I'm not yet sure if that's feasible or will actually be
> all that helpful so I'm happy to forgo that.)

Hmm, that sounds convenient for debugging, but affected cases would also 
be straightforward to debug by adding a run-time call, so I'm skeptical 
it would be worth the overhead for successful compiles.

Jason