* Add support and codepaths for C++20's [[no_unique_address]].

* Add many more tests for `detail::__compressed_pair`'s layout and ABI.

compilation_tests/CMakeLists.txt

@@ -10,6 +10,7 @@ add_compilation_test(ctest_mdspan_convertible)
 add_compilation_test(ctest_standard_layout)
 add_compilation_test(ctest_trivially_copyable)
 add_compilation_test(ctest_no_unique_address)
+add_compilation_test(ctest_compressed_pair_layout)
 add_compilation_test(ctest_constexpr_dereference)
 add_compilation_test(ctest_constexpr_submdspan)
 add_compilation_test(ctest_constexpr_layouts)

compilation_tests/ctest_compressed_pair_layout.cpp

@@ -0,0 +1,145 @@
+/*
+//@HEADER
+// ************************************************************************
+//
+//                        Kokkos v. 2.0
+//              Copyright (2020) Sandia Corporation
+//
+// Under the terms of Contract DE-AC04-94AL85000 with Sandia Corporation,
+// the U.S. Government retains certain rights in this software.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+// 1. Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright
+// notice, this list of conditions and the following disclaimer in the
+// documentation and/or other materials provided with the distribution.
+//
+// 3. Neither the name of the Corporation nor the names of the
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY SANDIA CORPORATION "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+// PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL SANDIA CORPORATION OR THE
+// CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// Questions? Contact Christian R. Trott (crtrott@sandia.gov)
+//
+// ************************************************************************
+//@HEADER
+*/
+
+#include "ctest_common.hpp"
+
+#include <experimental/mdspan>
+
+#include <type_traits>
+
+namespace stdex = std::experimental;
+
+enum emptyness {
+  non_empty = false,
+  empty     = true
+};
+
+enum standard_layoutness {
+  non_standard_layout = false,
+  standard_layout     = true
+};
+
+enum trivially_copyableness {
+  non_trivially_copyable = false,
+  trivially_copyable     = true
+};
+
+template <class T, size_t Size,
+          emptyness Empty = non_empty,
+          standard_layoutness StandardLayout = standard_layout,
+          trivially_copyableness TriviallyCopyable = trivially_copyable>
+void test() {
+  MDSPAN_STATIC_TEST(sizeof(T) == Size);
+  MDSPAN_STATIC_TEST(std::is_empty_v<T> == Empty);
+  MDSPAN_STATIC_TEST(std::is_standard_layout_v<T> == StandardLayout);
+  MDSPAN_STATIC_TEST(std::is_trivially_copyable_v<T> == TriviallyCopyable);
+}
+
+template <class T, class U>
+using CP = stdex::detail::__compressed_pair<T, U>;
+
+struct E0 {};
+struct E1 {};
+struct E2 {};
+struct E3 {};
+
+void instantiate_tests() {
+//==============================================================================
+// <editor-fold desc="compressed pair layout: 2 leaf elements"> {{{1
+test<CP<E0, E0>,     2,                empty>();
+test<CP<E0, E1>,     1,                empty>();
+test<CP<int*, E1>,   sizeof(int*),     non_empty>();
+test<CP<E0, int*>,   sizeof(int*),     non_empty>();
+test<CP<int*, int*>, 2 * sizeof(int*), non_empty>();
+// </editor-fold> end compressed pair layout: 2 leaf elements }}}1
+//==============================================================================
+
+//==============================================================================
+// <editor-fold desc="compressed pair layout: 1 nested pair, 3 leaf element"> {{{1
+#if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
+test<CP<E0,   CP<E0,   E0>>,   3,                empty>();     // Emulation can't handle this correctly.
+#endif
+test<CP<E0,   CP<E1,   E2>>,   1,                empty>();
+test<CP<E0,   CP<E1,   int*>>, sizeof(int*),     non_empty>();
+test<CP<E0,   CP<int*, E2>>,   sizeof(int*),     non_empty>();
+test<CP<E0,   CP<int*, int*>>, 2 * sizeof(int*), non_empty>();
+test<CP<int*, CP<E1,   E2>>,   sizeof(int*),     non_empty>();
+test<CP<int*, CP<E1,   int*>>, 2 * sizeof(int*), non_empty>();
+test<CP<int*, CP<int*, E2>>,   2 * sizeof(int*), non_empty>();
+test<CP<int*, CP<int*, int*>>, 3 * sizeof(int*), non_empty>();
+#if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
+test<CP<CP<E0,   E0>,   E0>,   3,                empty>();     // Emulation can't handle this correctly.
+#endif
+test<CP<CP<E0,   E1>,   E2>,   1,                empty>();
+test<CP<CP<E0,   E1>,   int*>, sizeof(int*),     non_empty>();
+test<CP<CP<E0,   int*>, E2>,   sizeof(int*),     non_empty>();
+test<CP<CP<E0,   int*>, int*>, 2 * sizeof(int*), non_empty>();
+test<CP<CP<int*, E1>,   E2>,   sizeof(int*),     non_empty>();
+test<CP<CP<int*, E1>,   int*>, 2 * sizeof(int*), non_empty>();
+test<CP<CP<int*, int*>, E2>,   2 * sizeof(int*), non_empty>();
+test<CP<CP<int*, int*>, int*>, 3 * sizeof(int*), non_empty>();
+// </editor-fold> end compressed pair layout: 1 nested pair, 3 leaf element }}}1
+//==============================================================================
+
+//==============================================================================
+// <editor-fold desc="compressed pair layout: 2 nested pairs, 4 leaf element"> {{{1
+#if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
+test<CP<CP<E0,   E0>,   CP<E0,  E0>>,    4,                empty>(); // Emulation can't handle this correctly.
+#endif
+test<CP<CP<E0,   E1>,   CP<E2,  E3>>,    1,                empty>();
+test<CP<CP<E0,   E1>,   CP<E2,   int*>>, sizeof(int*),     non_empty>();
+test<CP<CP<E0,   E1>,   CP<int*, E3>>,   sizeof(int*),     non_empty>();
+test<CP<CP<E0,   E1>,   CP<int*, int*>>, 2 * sizeof(int*), non_empty>();
+test<CP<CP<E0,   int*>, CP<E2,   int*>>, 2 * sizeof(int*), non_empty>();
+test<CP<CP<E0,   int*>, CP<int*, E3>>,   2 * sizeof(int*), non_empty>();
+test<CP<CP<E0,   int*>, CP<int*, int*>>, 3 * sizeof(int*), non_empty>();
+test<CP<CP<int*, E1>,   CP<E2,   int*>>, 2 * sizeof(int*), non_empty>();
+test<CP<CP<int*, E1>,   CP<int*, E3>>,   2 * sizeof(int*), non_empty>();
+test<CP<CP<int*, E1>,   CP<int*, int*>>, 3 * sizeof(int*), non_empty>();
+test<CP<CP<int*, int*>, CP<E2,   int*>>, 3 * sizeof(int*), non_empty>();
+test<CP<CP<int*, int*>, CP<int*, E3>>,   3 * sizeof(int*), non_empty>();
+test<CP<CP<int*, int*>, CP<int*, int*>>, 4 * sizeof(int*), non_empty>();
+// </editor-fold> end compressed pair layout: 2 nested pairs, 4 leaf elements }}}1
+//==============================================================================
+}
+

compilation_tests/ctest_standard_layout.cpp

@@ -52,27 +52,6 @@ namespace stdex = std::experimental;
 //==============================================================================
 // <editor-fold desc="helper utilities"> {{{1
 
-struct empty1 { };
-struct empty2 { };
-
-MDSPAN_STATIC_TEST(
-  std::is_standard_layout<
-    stdex::detail::__compressed_pair<empty1, empty2>
-  >::value
-);
-
-MDSPAN_STATIC_TEST(
-  std::is_standard_layout<
-    stdex::detail::__compressed_pair<int*, empty2>
-  >::value
-);
-
-MDSPAN_STATIC_TEST(
-  std::is_standard_layout<
-    stdex::detail::__compressed_pair<int*, stdex::detail::__compressed_pair<empty1, empty2>>
-  >::value
-);
-
 MDSPAN_STATIC_TEST(
   !std::is_base_of<stdex::extents<1, 2, 3>, stdex::detail::__partially_static_sizes<1, 2, 3>>::value
 );

compilation_tests/ctest_trivially_copyable.cpp

@@ -52,27 +52,6 @@ namespace stdex = std::experimental;
 //==============================================================================
 // <editor-fold desc="helper utilities"> {{{1
 
-struct empty1 { };
-struct empty2 { };
-
-MDSPAN_STATIC_TEST(
-  std::is_trivially_copyable<
-    stdex::detail::__compressed_pair<empty1, empty2>
-  >::value
-);
-
-MDSPAN_STATIC_TEST(
-  std::is_trivially_copyable<
-    stdex::detail::__compressed_pair<int*, empty2>
-  >::value
-);
-
-MDSPAN_STATIC_TEST(
-  std::is_trivially_copyable<
-    stdex::detail::__compressed_pair<int*, stdex::detail::__compressed_pair<empty1, empty2>>
-  >::value
-);
-
 MDSPAN_STATIC_TEST(
   !std::is_base_of<stdex::extents<1, 2, 3>, stdex::detail::__partially_static_sizes<1, 2, 3>>::value
 );

include/experimental/__p0009_bits/basic_mdspan.hpp

@@ -48,6 +48,7 @@
 #include "layout_right.hpp"
 #include "extents.hpp"
 #include "trait_backports.hpp"
+#include "compressed_pair.hpp"
 
 namespace std {
 namespace experimental {
@@ -77,10 +78,6 @@ class basic_mdspan<
 {
 private:
 
-  using __mapping_base_t = detail::__no_unique_address_emulation<
-    typename LayoutPolicy::template mapping<experimental::extents<Exts...>>, 0>;
-  using __accessor_base_t = detail::__no_unique_address_emulation<AccessorPolicy, 1>;
-
   // Workaround for non-deducibility of the index sequence template parameter if it's given at the top level
   template <class>
   struct __impl_impl;