[PATCH] libstdc++: Work around modules issue causing hello-1 ICE [PR113710]

[Patrick Palka <ppalka@redhat.com>]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

-- >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