* Add mdspan deduction guides for the array extents constructor and

  mapping/accessor constructors. Now all constructors have deduction guides.
* Improve the `mdspan` deduction guide for the constructor that takes a
  parameter pack of dynamic indices.
* Change `layout_stride::mapping`'s constructor to take a `dextents` for the
  strides instead of an `array`. This is both cleaner, and was needed to make
  CTAD work.
* Add a `layout` type alias to all layout mappings, which is needed by
  `mdspan`'s deduction guides for its layout mapping constructors.
* Move all `mdspan` initialization and CTAD tests into `test_mdspan_ctors`.
* Add `layout_(left|right)` and `layout_stride` initialization and CTAD tests to
  `test_layout_ctors` and `test_layout_stride` respectively.
* Add a test for `mdspan` construction from a C-array.
* Add a test for `mdspan` construction from an extents `array` of type
  convertible to, but not exactly, `size_t`.

include/experimental/__p0009_bits/basic_mdspan.hpp

@@ -330,10 +330,27 @@ class basic_mdspan<
 
 };
 
-#if _MDSPAN_USE_DEDUCTION_GUIDES
-template <class ElementType, class... Integrals>
-explicit basic_mdspan(ElementType*, Integrals...)
-  -> basic_mdspan<ElementType, extents<detail::__make_dynamic_extent<Integrals>()...>>;
+#if _MDSPAN_USE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+MDSPAN_TEMPLATE_REQUIRES(
+  class ElementType, class... SizeTypes,
+  /* requires */ _MDSPAN_FOLD_AND(_MDSPAN_TRAIT(is_integral, SizeTypes) /* && ... */)
+)
+explicit basic_mdspan(ElementType*, SizeTypes...)
+  -> basic_mdspan<ElementType, dextents<sizeof...(SizeTypes)>>;
+
+template <class ElementType, class SizeType, size_t N>
+explicit basic_mdspan(ElementType*, const array<SizeType, N>&)
+  -> basic_mdspan<ElementType, dextents<N>>;
+
+// TODO @proposal-bug Layout mappings in the proposal are not required to have `extents_type`.
+template <class ElementType, class MappingType>
+basic_mdspan(ElementType*, const MappingType&)
+  -> basic_mdspan<ElementType, typename MappingType::extents_type, typename MappingType::layout>;
+
+// TODO @proposal-bug Layout mappings in the proposal are not required to have `extents_type`.
+template <class ElementType, class MappingType, class AccessorType>
+basic_mdspan(ElementType*, const MappingType&, const AccessorType&)
+  -> basic_mdspan<ElementType, typename MappingType::extents_type, typename MappingType::layout, AccessorType>;
 #endif
 
 template <class T, size_t... Exts>

include/experimental/__p0009_bits/layout_left.hpp

@@ -69,7 +69,26 @@ struct layout_left_idx_conditional {
 struct layout_left {
   template <class Extents>
   class mapping {
+  public:
     static_assert(detail::__depend_on_instantiation_v<Extents, false>, "layout_left::mapping instantiated with invalid extents type.");
+
+#if _MDSPAN_USE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+    // GCC currently rejects deduction guides for member template classes at
+    // class scope, so we can't write the deduction guide we need for mappings.
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100983
+    //
+    // For this layout, we shouldn't need a deduction guide; the automatic ones
+    // should work. But, in our implementation we've chosen to leave the primary
+    // template empty and put the implementation in the specialization below.
+    // Unfortunately, it seems that the primary template is the one used for
+    // deduction.
+    //
+    // But, by cleverly adding a constructor declaration to the primary that
+    // serves as a pseudo deduction guide, we can get the automatic deduction
+    // guide to do the right thing.
+    MDSPAN_INLINE_FUNCTION
+    constexpr mapping(Extents const&) noexcept = delete;
+#endif
   };
 
   template <size_t... Exts>
@@ -97,6 +116,14 @@ struct layout_left {
 
     using base_t::base_t;
 
+    // TODO @proposal-bug This isn't a requirement of layouts in the proposal,
+    // but we need it for `mdspan`'s deduction guides for its mapping
+    // constructors.
+    using layout = layout_left;
+
+    // TODO @proposal-bug This isn't a requirement in the proposal.
+    using typename base_t::extents_type;
+
     // TODO noexcept specification
     MDSPAN_TEMPLATE_REQUIRES(
       class OtherExtents,
@@ -109,7 +136,6 @@ struct layout_left {
       : base_t(other.extents())
     { }
 
-
     // TODO noexcept specification
     MDSPAN_TEMPLATE_REQUIRES(
       class OtherExtents,
@@ -142,7 +168,7 @@ struct layout_left {
     }
 
   };
-}; 
+};
 
 } // end namespace experimental
 } // end namespace std

include/experimental/__p0009_bits/layout_right.hpp

@@ -69,7 +69,26 @@ struct layout_right_idx_conditional {
 struct layout_right {
   template <class Extents>
   class mapping {
+  public:
     static_assert(detail::__depend_on_instantiation_v<Extents, false>, "layout_right::mapping instantiated with invalid extents type.");
+
+#if _MDSPAN_USE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+    // GCC currently rejects deduction guides for member template classes at
+    // class scope, so we can't write the deduction guide we need for mappings.
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100983
+    //
+    // For this layout, we shouldn't need a deduction guide; the automatic ones
+    // should work. But, in our implementation we've chosen to leave the primary
+    // template empty and put the implementation in the specialization below.
+    // Unfortunately, it seems that the primary template is the one used for
+    // deduction.
+    //
+    // But, by cleverly adding a constructor declaration to the primary that
+    // serves as a pseudo deduction guide, we can get the automatic deduction
+    // guide to do the right thing.
+    MDSPAN_INLINE_FUNCTION
+    constexpr mapping(Extents const&) noexcept = delete;
+#endif
   };
 
   template <size_t... Exts>
@@ -97,6 +116,14 @@ struct layout_right {
 
     using base_t::base_t;
 
+    // TODO @proposal-bug This isn't a requirement of layouts in the proposal,
+    // but we need it for `mdspan`'s deduction guides for its mapping
+    // constructors.
+    using layout = layout_right;
+
+    // TODO @proposal-bug This isn't a requirement in the proposal.
+    using typename base_t::extents_type;
+
     // TODO noexcept specification
     MDSPAN_TEMPLATE_REQUIRES(
       class OtherExtents,

include/experimental/__p0009_bits/layout_stride.hpp

@@ -66,12 +66,24 @@ struct layout_stride {
   template <class Extents>
   class mapping {
     static_assert(detail::__depend_on_instantiation_v<Extents, false>, "layout_stride::mapping instantiated with invalid extents type.");
+
+#if _MDSPAN_USE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+    // GCC currently rejects deduction guides for member template classes at
+    // class scope, so we can't write the deduction guide we need for mappings.
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100983
+    //
+    // (Un)fortunately, with an oh-so-clever constructor declaration in the
+    // primary that serves as a pseudo deduction guide, we can get the
+    // automatic deduction guide to do the right thing.
+    MDSPAN_INLINE_FUNCTION
+    constexpr mapping(Extents const&, dextents<Extents::rank()> const&) noexcept = delete;
+#endif
   };
 
   template <size_t... Exts>
   class mapping<
     std::experimental::extents<Exts...>
-  > 
+  >
 #if !defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
     : private detail::__no_unique_address_emulation<
         detail::__compressed_pair<
@@ -85,6 +97,11 @@ struct layout_stride {
 
     using extents_type = experimental::extents<Exts...>;
 
+    // TODO @proposal-bug This isn't a requirement of layouts in the proposal,
+    // but we need it for `mdspan`'s deduction guides for its mapping
+    // constructors.
+    using layout = layout_stride;
+
   private:
 
     using idx_seq = make_index_sequence<sizeof...(Exts)>;
@@ -217,19 +234,21 @@ struct layout_stride {
     mapping& operator=(mapping&&) noexcept = default;
     MDSPAN_INLINE_FUNCTION_DEFAULTED ~mapping() noexcept = default;
 
-    // TODO @proposal-bug layout stride needs this constructor
+    // TODO @proposal-bug In the proposal, the constructor doesn't take a
+    // `dextents`, and doesn't accept any `array` with elements convertible to
+    // `size_t`.
     MDSPAN_INLINE_FUNCTION
     constexpr
     mapping(
       std::experimental::extents<Exts...> const& e,
-      array<size_t, __strides_storage_t::rank()> const& strides
+      std::experimental::dextents<__strides_storage_t::rank()> const& strides
     ) noexcept
 #if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
       : __members{
 #else
       : __base_t(__base_t{__member_pair_t(
 #endif
-          e, __strides_storage_t(strides)
+          e, __strides_storage_t{strides}
 #if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
         }
 #else
@@ -243,7 +262,7 @@ struct layout_stride {
     MDSPAN_INLINE_FUNCTION constexpr extents_type extents() const noexcept {
 #if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
       return __members.__first();
-#else 
+#else
       return this->__base_t::__ref().__first();
 #endif
     };

include/experimental/__p0009_bits/macros.hpp

@@ -235,7 +235,7 @@
 #else
 #  define _MDSPAN_DEDUCE_RETURN_TYPE_SINGLE_LINE(SIGNATURE, BODY) \
     auto MDSPAN_PP_REMOVE_PARENS(SIGNATURE) \
-      -> typename std::remove_cv<typename std::remove_reference<decltype(BODY)>::type>::type \
+      -> std::remove_cv_t<std::remove_reference_t<decltype(BODY)>> \
     { return MDSPAN_PP_REMOVE_PARENS(BODY); }
 #  define _MDSPAN_DEDUCE_DECLTYPE_AUTO_RETURN_TYPE_SINGLE_LINE(SIGNATURE, BODY) \
     auto MDSPAN_PP_REMOVE_PARENS(SIGNATURE) \

tests/CMakeLists.txt

@@ -35,9 +35,9 @@ add_subdirectory(${CMAKE_CURRENT_BINARY_DIR}/googletest-src
 
 mdspan_add_test(test_extents)
 mdspan_add_test(test_contiguous_layouts)
-mdspan_add_test(test_ctors)
+mdspan_add_test(test_mdspan_ctors)
+mdspan_add_test(test_mdspan_conversion)
 mdspan_add_test(test_layout_ctors)
 mdspan_add_test(test_layout_stride)
 mdspan_add_test(test_element_access)
-mdspan_add_test(test_conversion)
 

tests/test_contiguous_layouts.cpp

@@ -166,67 +166,3 @@ TYPED_TEST(TestLayout3D, mapping_works) {
 TYPED_TEST(TestLayout3D, required_span_size_works) {
   ASSERT_EQ(this->map.required_span_size(), 3*4*5);
 }
-
-TEST(TestLayoutLeftListInitialization, test_layout_left_list_initialization) {
-  double* data = nullptr;
-  stdex::basic_mdspan<double, stdex::extents<dyn, dyn>, stdex::layout_left> m(data, {{16, 32}});
-  ASSERT_EQ(m.rank(), 2);
-  ASSERT_EQ(m.rank_dynamic(), 2);
-  ASSERT_EQ(m.stride(0), 1);
-  ASSERT_EQ(m.stride(1), 16);
-  ASSERT_EQ(m.extent(0), 16);
-  ASSERT_EQ(m.extent(1), 32);
-  ASSERT_TRUE(m.is_contiguous());
-}
-
-// GCC 10 ICEs if I stick this in the body of the below test.
-template <typename M>
-void test_layout_left_ctad_gcc_10_workaround(M m) {
-  ASSERT_EQ(m.rank(), 2);
-  ASSERT_EQ(m.rank_dynamic(), 2);
-  ASSERT_EQ(m.stride(0), 1);
-  ASSERT_EQ(m.stride(1), 32);
-  ASSERT_EQ(m.extent(0), 16);
-  ASSERT_EQ(m.extent(1), 32);
-}
-
-// TODO: this fails to compile on GCC 9.2 and clang 12 and likely others
-/*
-TEST(TestLayoutLeftCTAD, test_layout_left_ctad) {
-  double* data = nullptr;
-  stdex::basic_mdspan m(data, stdex::layout_left::mapping{stdex::extents{16, 32}});
-  test_layout_left_ctad_gcc_10_workaround(m);
-}
-*/
-
-TEST(TestLayoutRightListInitialization, test_layout_right_list_initialization) {
-  double* data = nullptr;
-  stdex::basic_mdspan<double, stdex::extents<dyn, dyn>, stdex::layout_right> m(data, {{16, 32}});
-  ASSERT_EQ(m.rank(), 2);
-  ASSERT_EQ(m.rank_dynamic(), 2);
-  ASSERT_EQ(m.stride(0), 32);
-  ASSERT_EQ(m.stride(1), 1);
-  ASSERT_EQ(m.extent(0), 16);
-  ASSERT_EQ(m.extent(1), 32);
-  ASSERT_TRUE(m.is_contiguous());
-}
-
-// GCC 10 ICEs if I stick this in the body of the below test.
-template <typename M>
-void test_layout_right_ctad_gcc_10_workaround(M m) {
-  ASSERT_EQ(m.rank(), 2);
-  ASSERT_EQ(m.rank_dynamic(), 2);
-  ASSERT_EQ(m.stride(0), 1);
-  ASSERT_EQ(m.stride(1), 32);
-  ASSERT_EQ(m.extent(0), 16);
-  ASSERT_EQ(m.extent(1), 32);
-}
-
-// This fails on GCC 9.2 and clang 12 and likely others
-/*
-TEST(TestLayoutRightCTAD, test_layout_right_ctad) {
-  double* data = nullptr;
-  stdex::basic_mdspan m(data, stdex::layout_right::mapping{stdex::extents{16, 32}});
-  test_layout_right_ctad_gcc_10_workaround(m);
-}
-*/

tests/test_extents.cpp

@@ -176,6 +176,15 @@ TYPED_TEST(TestExtentsCompatCtors, compatible_assign_2) {
   EXPECT_EQ(this->exts1, this->exts2);
 }
 
+TEST(TestExtentsCtorStdArrayConvertibleToSizeT, test_extents_ctor_std_array_convertible_to_size_t) {
+  std::array<int, 2> i{2, 2};
+  stdex::dextents<2> e{i};
+  ASSERT_EQ(e.rank(), 2);
+  ASSERT_EQ(e.rank_dynamic(), 2);
+  ASSERT_EQ(e.extent(0), 2);
+  ASSERT_EQ(e.extent(1), 2);
+}
+
 #if _MDSPAN_USE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 TEST(TestExtentsCTADPack, test_extents_ctad_pack) {
   stdex::extents m0;