From 63bda370eddf091c456cea3df82e0e5aa9a1fa97 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 22 Apr 2025 12:55:05 -0400
Subject: [PATCH] Revert "libstdc++: Optimize std::projected<I, std::identity>"
 [PR119888]

This non-standard optimization breaks real-world code that expects the
result of std::projected to always (be a class type and) have a value_type
member, which isn't true for e.g. I=int*, so revert it for now.

	PR libstdc++/119888

This reverts commit 51761c50f843d5be4e24172535e4524b5072f24c.
---
 libstdc++-v3/include/bits/iterator_concepts.h                | 5 -----
 .../testsuite/24_iterators/indirect_callable/projected.cc    | 5 -----
 2 files changed, 10 deletions(-)

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index e36556dc5121..3b73ff9b6b59 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -829,11 +829,6 @@ _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
diff --git a/libstdc++-v3/testsuite/24_iterators/indirect_callable/projected.cc b/libstdc++-v3/testsuite/24_iterators/indirect_callable/projected.cc
index 0b18616aa8b7..e2fbf7dccb9c 100644
--- a/libstdc++-v3/testsuite/24_iterators/indirect_callable/projected.cc
+++ b/libstdc++-v3/testsuite/24_iterators/indirect_callable/projected.cc
@@ -22,12 +22,7 @@
 template<typename T>
   using PI = std::projected<T, std::identity>;
 
-#if __GLIBCXX__
-// Verify our projected<I, identity> optimization.
-static_assert(std::same_as<PI<int*>, int*>);
-#else
 static_assert(std::same_as<PI<int*>::value_type, int>);
-#endif
 static_assert(std::same_as<decltype(*std::declval<const PI<int*>&>()), int&>);
 
 struct X
-- 
2.47.2