On Wed, 7 Feb 2024, Jonathan Wakely wrote:

> 
> 
> On Wed, 7 Feb 2024 at 17:29, Patrick Palka <ppalka@redhat.com> wrote:
>       Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> 
> 
> OK.
> 
> Do we have a reduced testcase to track the modules bug that will still exist in the FE?

https://gcc.gnu.org/PR113814

> 
>  
> 
>       -- >8 --
> 
>       The forward declarations of std::get in <bits/stl_pair.h> added in
>       r14-8710-g65b4cba9d6a9ff are causing an ICE in the test modules/hello-1
>       due to what seems to be a declaration merging issue in modules.
> 
>       What seems to be happening is that in hello-1_b.C we first include
>       <string_view>, which indirectly includes <bits/stl_pair.h> which forms
>       the dependent specialization tuple_element<__i, tuple<_Elements...>>
>       (appearing in some of the std::get overloads) and adds it to the
>       specializations table.
> 
>       We then import hello which indirectly includes <tuple> (in the GMF),
>       within which we define a partial specialization of tuple_element with
>       that same template-id.  So importing hello in turn streams in this
>       partial specialization, but we don't notice it matches the previously
>       created dependent specialization, and we end up with two equivalent
>       types for this template-id with different TYPE_CANONICAL.
> 
>       This patch works around this issue by adding a forward declaration of
>       the tuple_element partial specialization from <tuple> to <bits/stl_pair.h>
>       so that it appears alongside the dependent specialization of the same
>       template-id.  So when including <bits/stl_pair.h> we immediately register
>       the template-id as a partial specialization, and if we later stream in the
>       partial specialization the MK_partial case of trees_in::key_mergeable will
>       match them up.  (So perhaps a proper modules fix for this might be to make
>       key_mergeable try to match up a streamed in partial specialization with an
>       existing specialization from the table via match_mergeable_specialization.)
> 
>               PR c++/113710
> 
>       libstdc++-v3/ChangeLog:
> 
>               * include/bits/stl_pair.h (tuple_element): Add forward
>               declaration of the partial specialization for tuple.
>       ---
>        libstdc++-v3/include/bits/stl_pair.h | 5 +++++
>        1 file changed, 5 insertions(+)
> 
>       diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
>       index 00ec53ebc33..8c71b1350e5 100644
>       --- a/libstdc++-v3/include/bits/stl_pair.h
>       +++ b/libstdc++-v3/include/bits/stl_pair.h
>       @@ -1153,6 +1153,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>            struct tuple_element<1, pair<_Tp1, _Tp2>>
>            { typedef _Tp2 type; };
> 
>       +  // Forward declare the partial specialization for std::tuple
>       +  // to work around modules bug PR c++/113710.
>       +  template<size_t __i, typename... _Types>
>       +    struct tuple_element<__i, tuple<_Types...>>;
>       +
>        #if __cplusplus >= 201703L
>          template<typename _Tp1, typename _Tp2>
>            inline constexpr size_t tuple_size_v<pair<_Tp1, _Tp2>> = 2;
>       --
>       2.43.0.561.g235986be82
> 
> 
>