Re: [PATCH 1/2] c++: potentiality of templated memfn call [PR109480]

[Jason Merrill <jason@redhat.com>]

On 5/12/23 13:02, Patrick Palka wrote:
> Hi Martin,
> 
> On Fri, May 12, 2023 at 12:13 PM Martin Jambor <mjambor@suse.cz> wrote:
>>
>> Hello Patrick,
>>
>> On Wed, May 03 2023, Patrick Palka via Gcc-patches wrote:
>>>
>> [...]
>>>
>>> Subject: [PATCH] c++: potentiality of templated memfn call [PR109480]
>>>
>>> Here we're incorrectly deeming the templated call a.g() inside b's
>>> initializer as potentially constant, despite g being non-constexpr,
>>> which leads to us wastefully instantiating the initializer ahead of time,
>>> which incidentally tiggers a bug in access checking deferral (to be
>>> fixed by the subsequent patch).
>>>
>>> This patch fixes this by calling get_fns earlier during CALL_EXPR
>>> potentiality checking so that we're able to extract a FUNCTION_DECL out
>>> of a templated member function call (whose overall is typically a
>>> COMPONENT_REF) and to the usual checking if the called function is
>>> constexpr etc.
>>>
>>> In passing, I noticed potential_constant_expression_1's special handling
>>> of the object argument of a non-static member function call is effectively
>>> the same as the generic argument handling a few lines later.  So this
>>> patch just gets rid of this special handling; otherwise we'd have to adapt
>>> it to handle templated versions of such calls.
>>>
>>>        PR c++/109480
>>>
>>> gcc/cp/ChangeLog:
>>>
>>>        * constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
>>>        Reorganize to call get_fns sooner.  Remove special handling of
>>>        the object argument of a non-static member function call.  Remove
>>>        dead store to 'fun'.
>>>
>>
>> This patch makes g++ no longer accept the following, complaining that
>> get_subsys is non-constexpr (with just -std=c++17 -S), which is of
>> course auto-reduced from a much larger source file from Ceph:
>>
>> ----------------------------------- 8< -----------------------------------
>> struct {
>>    void get_subsys();
>> } PriorSet_dpp;
>> struct PriorSet {
>>    template <typename> PriorSet();
>> };
>> template <typename> PriorSet::PriorSet() {
>>    [](auto cctX) { cctX.template should_gather<PriorSet_dpp.get_subsys()>; };
>> }
>> ----------------------------------- 8< -----------------------------------
>>
>> I assume that is intentional and am actually somewhat surprised it was
>> accepted before, but can you please confirm?
> 
> Yes, this seems correct/intentional to me-- no instantiation of the
> template would be valid because it's trying to use a non-constant
> expression (which we now correctly identify as such) as a template
> argument, so this snippet is IFNDR.
> 
> I don't think we have testsuite coverage for this QoI diagnostic, I'll add one.

Incidentally, I wonder about trying to make IFNDR diags in general 
permerrors or default-error pedwarns, but that doesn't need to happen now.

Jason