[PATCH] c++/modules: Ensure deduction guides are always reachable [PR115231]

[Jason Merrill]

On 7/26/24 12:52 AM, Nathaniel Shead wrote:
> On Tue, Jul 23, 2024 at 04:17:22PM -0400, Jason Merrill wrote:
>> On 6/15/24 10:29 PM, Nathaniel Shead wrote:
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
>>>
>>> This probably isn't the most efficient approach, since we need to do
>>> name lookup to find deduction guides for a type which will also
>>> potentially do a bunch of pointless lazy loading from imported modules,
>>> but I wasn't able to work out a better approach without completely
>>> reworking how deduction guides are stored and represented.
>>
>> Indeed.  We likely want to find them more directly from the template; it's
>> not clear to me that DECL_INITIAL is used for TEMPLATE_DECL, or we could put
>> them in an internal attribute or a separate hash table.
>>
>>> -- >8 --
>>>
>>> Deduction guides are represented as 'normal' functions currently, and
>>> have no special handling in modules.  However, this causes some issues;
>>> by [temp.deduct.guide] a deduction guide is not found by normal name
>>> lookup and instead all reachable deduction guides for a class template
>>> should be considered, but this does not happen currently.
>>>
>>> To solve this, this patch ensures that all deduction guides are
>>> considered exported to ensure that they are always visible to importers
>>> if they are reachable.  Another alternative here would be to add a new
>>> kind of "all reachable" flag to name lookup, but that is complicated by
>>> some difficulties in handling GM entities; this may be a better way to
>>> go if more kinds of entities end up needing this handling, however.
>>>
>>> Another issue here is that because deduction guides are "unrelated"
>>> functions, they will usually get discarded from the GMF, so this patch
>>> ensures that when finding dependencies, GMF deduction guides will also
>>> have bindings created.  We do this in find_dependencies so that we don't
>>> unnecessarily create bindings for GMF deduction guides that are never
>>> reached; for consistency we do this for *all* deduction guides, not just
>>> GM ones.
>>
>> If you fixed the dependency calculation, why do they also need to be
>> exported?
> 
> Deduction guides aren't found using normal name lookup, but any
> reachable deduction guide must be considered.  This means that even if
> the module interface exports no declarations whatsoever, a deduction
> guide declared in the module purview must still be considered by
> importers.

Ah, I was missing the name lookup issue.

> The other option I've considered is adding a new "ANY_REACHABLE" flag to
> name lookup which would also consider non-exported reachable decls.  On
> further consideration I might actually go this way; I've been thinking
> about how to resolve some issues adjacent to supporting textual
> redefinitions that I believe this will be necessary for anyway, and we
> can probably use this in tsubst_friend_class as well rather than the
> current relatively ad-hoc solution.

There's also my hack in lookup_elaborated_type for ABI namespace types.

I'm not sure that it should be necessary to do this for redefinitions, 
though; what's the advantage over merging in check_module_override 
(apart from that needing to be fixed)?

> That said, I've realised that this patch isn't completely sufficient
> anyway; consider:
> 
>    // m.cpp
>    module;
>    template <typename T> struct S;
>    export module M;
>    S(int) -> S<double>;
> 
>    // x.cpp
>    template <typename T> struct S { S(int); };
>    import M;
>    int main() {
>      S s(10);  // should be S<double> s;
>    }
> 
> This patch doesn't correctly handle this case yet, we need to also
> consider cases where only the deduction guide is in purview.

Indeed.

Jason