Re: [PATCH] c++/modules: Prefer partition indexes when installing imported entities [PR99377]

[Jason Merrill <jason@redhat.com>]

On 3/28/24 08:22, Nathaniel Shead wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> 
> -- >8 --
> 
> The testcase in comment 15 of the linked PR is caused because the
> following assumption in depset::hash::make_dependency doesn't hold:
> 
>    if (DECL_LANG_SPECIFIC (not_tmpl)
>        && DECL_MODULE_IMPORT_P (not_tmpl))
>      {
>        /* Store the module number and index in cluster/section,
>           so we don't have to look them up again.  */
>        unsigned index = import_entity_index (decl);
>        module_state *from = import_entity_module (index);
>        /* Remap will be zero for imports from partitions, which
>           we want to treat as-if declared in this TU.  */
>        if (from->remap)
>          {
>            dep->cluster = index - from->entity_lwm;
>            dep->section = from->remap;
>            dep->set_flag_bit<DB_IMPORTED_BIT> ();
>          }
>      }
> 
> This is because at least for template specialisations, we first see the
> declaration in the header unit imported from the partition, and then the
> instantiation provided by the partition itself.  This means that the
> 'import_entity_index' lookup doesn't report that the specialisation was
> declared in the partition and thus should be considered as-if it was
> part of the TU, and get exported.

I think "exported" is the wrong term here; IIUC template specializations 
are not themselves exported, just the template itself.

But if the declaration or point of instantiation of the specialization 
is within a module instantiation unit, it is reachable to any importers, 
including the primary module interface unit importing the partition 
interface unit.

Does this work differently if "check" is a separate module rather than a 
partition?

> To fix this, this patch allows, as a special case for installing an
> entity from a partition, to overwrite the entity_map entry with the
> (later) index into the partition so that this assumption holds again.

Rather than special-casing partitions, would it make sense to override a 
declaration with a definition?

Jason