[libc++] Fix uses of non-empty transparent comparator in <map> (#152624)

The `__get_value()` member function was removed in LLVM 21, but the
calls in `<map>` weren't removed. This patch completes the removal and
adds regression test cases.

Fixes #152543.

(cherry picked from commit 7ae1424286203e37f6238e9349b1bfbe873ebabf)

libcxx/include/map

@@ -692,12 +692,12 @@ public:
 #  if _LIBCPP_STD_VER >= 14
   template <typename _K2>
   _LIBCPP_HIDE_FROM_ABI bool operator()(const _K2& __x, const _CP& __y) const {
-    return __comp_(__x, __y.__get_value().first);
+    return __comp_(__x, __y.first);
   }
 
   template <typename _K2>
   _LIBCPP_HIDE_FROM_ABI bool operator()(const _CP& __x, const _K2& __y) const {
-    return __comp_(__x.__get_value().first, __y);
+    return __comp_(__x.first, __y);
   }
 #  endif
 };

libcxx/test/std/containers/associative/map/map.ops/count0.pass.cpp

@@ -33,6 +33,10 @@ int main(int, char**) {
     typedef std::map<int, double, transparent_less_not_referenceable> M;
     assert(M().count(C2Int{5}) == 0);
   }
+  {
+    using M = std::map<int, double, transparent_less_nonempty>;
+    assert(M().count(C2Int{5}) == 0);
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/map/map.ops/equal_range0.pass.cpp

@@ -40,6 +40,13 @@ int main(int, char**) {
     P result = example.equal_range(C2Int{5});
     assert(result.first == result.second);
   }
+  {
+    using M = std::map<int, double, transparent_less_nonempty>;
+    using P = std::pair<typename M::iterator, typename M::iterator>;
+    M example;
+    P result = example.equal_range(C2Int{5});
+    assert(result.first == result.second);
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/map/map.ops/find0.pass.cpp

@@ -36,6 +36,11 @@ int main(int, char**) {
     M example;
     assert(example.find(C2Int{5}) == example.end());
   }
+  {
+    using M = std::map<int, double, transparent_less_nonempty>;
+    M example;
+    assert(example.find(C2Int{5}) == example.end());
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/map/map.ops/lower_bound0.pass.cpp

@@ -36,6 +36,11 @@ int main(int, char**) {
     M example;
     assert(example.lower_bound(C2Int{5}) == example.end());
   }
+  {
+    using M = std::map<int, double, transparent_less_nonempty>;
+    M example;
+    assert(example.lower_bound(C2Int{5}) == example.end());
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/map/map.ops/upper_bound0.pass.cpp

@@ -36,6 +36,11 @@ int main(int, char**) {
     M example;
     assert(example.upper_bound(C2Int{5}) == example.end());
   }
+  {
+    using M = std::map<int, double, transparent_less_nonempty>;
+    M example;
+    assert(example.upper_bound(C2Int{5}) == example.end());
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/multimap/multimap.ops/count0.pass.cpp

@@ -33,6 +33,10 @@ int main(int, char**) {
     typedef std::multimap<int, double, transparent_less_not_referenceable> M;
     assert(M().count(C2Int{5}) == 0);
   }
+  {
+    using M = std::multimap<int, double, transparent_less_nonempty>;
+    assert(M().count(C2Int{5}) == 0);
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/multimap/multimap.ops/equal_range0.pass.cpp

@@ -40,6 +40,13 @@ int main(int, char**) {
     P result = example.equal_range(C2Int{5});
     assert(result.first == result.second);
   }
+  {
+    using M = std::multimap<int, double, transparent_less_nonempty>;
+    using P = std::pair<typename M::iterator, typename M::iterator>;
+    M example;
+    P result = example.equal_range(C2Int{5});
+    assert(result.first == result.second);
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/multimap/multimap.ops/find0.pass.cpp

@@ -36,6 +36,11 @@ int main(int, char**) {
     M example;
     assert(example.find(C2Int{5}) == example.end());
   }
+  {
+    using M = std::multimap<int, double, transparent_less_nonempty>;
+    M example;
+    assert(example.find(C2Int{5}) == example.end());
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/multimap/multimap.ops/lower_bound0.pass.cpp

@@ -36,6 +36,11 @@ int main(int, char**) {
     M example;
     assert(example.lower_bound(C2Int{5}) == example.end());
   }
+  {
+    using M = std::multimap<int, double, transparent_less_nonempty>;
+    M example;
+    assert(example.lower_bound(C2Int{5}) == example.end());
+  }
 
   return 0;
 }

libcxx/test/std/containers/associative/multimap/multimap.ops/upper_bound0.pass.cpp

@@ -36,6 +36,11 @@ int main(int, char**) {
     M example;
     assert(example.upper_bound(C2Int{5}) == example.end());
   }
+  {
+    using M = std::multimap<int, double, transparent_less_nonempty>;
+    M example;
+    assert(example.upper_bound(C2Int{5}) == example.end());
+  }
 
   return 0;
 }

libcxx/test/support/is_transparent.h

@@ -36,6 +36,17 @@ struct transparent_less_not_referenceable
     using is_transparent = void () const &;  // it's a type; a weird one, but a type
 };
 
+// Prevent regression when empty base class optimization is not suitable.
+// See https://github.com/llvm/llvm-project/issues/152543.
+struct transparent_less_nonempty {
+  template <class T, class U>
+  constexpr bool operator()(T&& t, U&& u) const {
+    return std::forward<T>(t) < std::forward<U>(u);
+  }
+  struct is_transparent {
+  } pad_; // making this comparator non-empty
+};
+
 struct transparent_less_no_type
 {
     template <class T, class U>