[PATCH 3/2] libstdc++: Optimize std::projected<I, std::identity>

[Patrick Palka]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?  I'm not
sure if the current specification of 'projected' strictly speaking
allows for this optimization, but it seems like a natural one that
should be allowed.

-- >8 --

Algorithms that are generalized to take projections usually default the
projection to std::identity, which really means no projection at all.
In that case, I believe we could shortcut the projection logic to return
the indirectly readable type unchanged rather than a wrapped version of
it.  This should help with compile times especially after P2609R3 which
made the indirect invocability concepts more expensive to check when
projections are in the picture.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (__detail::__projected):
	Define partial specialization for a std::identity projection.
---
 libstdc++-v3/include/bits/iterator_concepts.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index d849ddc32fc..642c709fee0 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -803,6 +803,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  using __projected_Proj = _Proj;
 	};
       };
+
+    // Optimize the common case of the projection being std::identity.
+    template<typename _Iter>
+      struct __projected<_Iter, identity>
+      { using __type = _Iter; };
   } // namespace __detail
 
   /// [projected], projected
-- 
2.46.0.267.gbb9c16bd4f