initial impl of P1976R2

include/gsl/span

@@ -420,18 +420,28 @@ public:
     constexpr span() noexcept : storage_(nullptr, details::extent_type<0>())
     {}
 
+     template<std::size_t extent = Extent, std::enable_if_t<extent != gsl::dynamic_extent, int> = 0>
     constexpr span(pointer ptr, size_type count) noexcept : storage_(ptr, count)
     {
-        if (Extent != dynamic_extent) Expects(count == Extent);
+        Expects(count == Extent);
     }
 
+     template<std::size_t extent = Extent, std::enable_if_t<extent == gsl::dynamic_extent, int> = 0>
+    constexpr explicit span(pointer ptr, size_type count) noexcept : storage_(ptr, count)
+    {}
+
+    template<std::size_t extent = Extent, std::enable_if_t<extent != gsl::dynamic_extent, int> = 0>
     constexpr span(pointer firstElem, pointer lastElem) noexcept
         : storage_(firstElem, static_cast<std::size_t>(lastElem - firstElem))
     {
-        if (Extent != dynamic_extent)
+        Expects(lastElem - firstElem == static_cast<difference_type>(Extent));
-        { Expects(lastElem - firstElem == static_cast<difference_type>(Extent)); }
     }
 
+    template<std::size_t extent = Extent, std::enable_if_t<extent == gsl::dynamic_extent, int> = 0>
+    constexpr span(pointer firstElem, pointer lastElem) noexcept
+        : storage_(firstElem, static_cast<std::size_t>(lastElem - firstElem))
+    {}
+
     template <std::size_t N,
               std::enable_if_t<details::is_allowed_extent_conversion<N, Extent>::value, int> = 0>
     constexpr span(element_type (&arr)[N]) noexcept
@@ -482,14 +492,25 @@ public:
     constexpr span(const span& other) noexcept = default;
 
     template <
-        class OtherElementType, std::size_t OtherExtent,
+        class OtherElementType, std::size_t OtherExtent, std::size_t MyExtent = Extent,
-        class = std::enable_if_t<
+        std::enable_if_t<
-            details::is_allowed_extent_conversion<OtherExtent, Extent>::value &&
+            !(MyExtent != dynamic_extent && OtherExtent == dynamic_extent) &&
-            details::is_allowed_element_type_conversion<OtherElementType, element_type>::value>>
+            (Extent == dynamic_extent || OtherExtent == dynamic_extent || Extent == OtherExtent) &&
+            details::is_allowed_element_type_conversion<OtherElementType, element_type>::value, int> = 0>
     constexpr span(const span<OtherElementType, OtherExtent>& other) noexcept
         : storage_(other.data(), details::extent_type<OtherExtent>(other.size()))
     {}
 
+    template <
+        class OtherElementType, std::size_t OtherExtent, std::size_t MyExtent = Extent,
+        std::enable_if_t<
+            (MyExtent != dynamic_extent && OtherExtent == dynamic_extent) &&
+            (Extent == dynamic_extent || OtherExtent == dynamic_extent || Extent == OtherExtent) &&
+            details::is_allowed_element_type_conversion<OtherElementType, element_type>::value, int> = 0>
+    constexpr explicit span(const span<OtherElementType, OtherExtent>& other) noexcept
+        : storage_(other.data(), details::extent_type<OtherExtent>(other.size()))
+    {}
+
     ~span() noexcept = default;
     constexpr span& operator=(const span& other) noexcept = default;
 
@@ -519,14 +540,14 @@ public:
         typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
         Expects((size() >= Offset) && (Count == dynamic_extent || (Count <= size() - Offset)));
-
+        using type = typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type;
-        return {data() + Offset, Count == dynamic_extent ? size() - Offset : Count};
+        return type(data() + Offset, Count == dynamic_extent ? size() - Offset : Count);
     }
 
     constexpr span<element_type, dynamic_extent> first(size_type count) const noexcept
     {
         Expects(count <= size());
-        return {data(), count};
+        return span<element_type>(data(), count);
     }
 
     constexpr span<element_type, dynamic_extent> last(size_type count) const noexcept
@@ -731,7 +752,8 @@ as_bytes(span<ElementType, Extent> s) noexcept
     // clang-format off
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
     // clang-format on
-    return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
+    return span<const byte, details::calculate_byte_size<ElementType, Extent>::value>(
+        reinterpret_cast<const byte*>(s.data()), s.size_bytes());
 }
 
 template <class ElementType, std::size_t Extent,
@@ -742,7 +764,8 @@ as_writable_bytes(span<ElementType, Extent> s) noexcept
     // clang-format off
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
     // clang-format on
-    return {reinterpret_cast<byte*>(s.data()), s.size_bytes()};
+    return span<byte, details::calculate_byte_size<ElementType, Extent>::value>(
+        reinterpret_cast<byte*>(s.data()), s.size_bytes());
 }
 
 } // namespace gsl

include/gsl/string_span

@@ -121,7 +121,7 @@ span<T, dynamic_extent> ensure_sentinel(T* seq,
     GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute // TODO: suppress does not work
     while (static_cast<std::size_t>(cur - seq) < max && *cur != Sentinel) ++cur;
     Ensures(*cur == Sentinel);
-    return {seq, static_cast<std::size_t>(cur - seq)};
+    return span<T>(seq, static_cast<std::size_t>(cur - seq));
 }
 
 //
@@ -308,7 +308,7 @@ public:
 private:
     static impl_type remove_z(pointer const& sz, std::size_t max)
     {
-        return {sz, details::string_length(sz, max)};
+        return impl_type(sz, details::string_length(sz, max));
     }
 
     template <std::size_t N>

tests/span_compatibility_tests.cpp

@@ -846,8 +846,6 @@ static_assert(!std::is_constructible<gsl::span<int, 3>, std::array<int, 500>&>::
               "!std::is_constructible<gsl::span<int, 3>, std::array<int, 500>&>");
 static_assert(!std::is_constructible<gsl::span<int, 3>, const std::array<int, 3>&>::value,
               "!std::is_constructible<gsl::span<int, 3>, const std::array<int, 3>&>");
-static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<int>&>::value,
-              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<int>&>");
 static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<int, 500>&>::value,
               "!std::is_constructible<gsl::span<int, 3>, const gsl::span<int, 500>&>");
 static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int>&>::value,
@@ -866,12 +864,8 @@ static_assert(!std::is_constructible<gsl::span<const int>, std::array<double, 3>
 static_assert(!std::is_constructible<gsl::span<const int>, const gsl::span<double, 3>&>::value,
               "!std::is_constructible<gsl::span<const int>, const gsl::span<double, 3>&>");
 
-static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int>&>::value,
-              "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int>&>");
 static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int, 500>&>::value,
               "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int, 500>&>");
-static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int>&>::value,
-              "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int>&>");
 static_assert(
     !std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int, 500>&>::value,
     "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int, 500>&>");
@@ -925,6 +919,14 @@ static_assert(!std::is_constructible<gsl::span<const Derived>, std::array<Base,
 static_assert(!std::is_constructible<gsl::span<const Derived>, const std::array<Base, 3>&>::value,
               "!std::is_constructible<gsl::span<const Derived>, const std::array<Base, 3>&>");
 
+// Explicit construction enabled in P1976R2
+static_assert(std::is_constructible<gsl::span<int, 3>, const gsl::span<int>&>::value,
+              "std::is_constructible<gsl::span<int, 3>, const gsl::span<int>&>");
+static_assert(std::is_constructible<gsl::span<const int, 3>, const gsl::span<int>&>::value,
+              "std::is_constructible<gsl::span<const int, 3>, const gsl::span<int>&>");
+static_assert(std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int>&>::value,
+              "std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int>&>");
+
 // no throw copy constructor
 static_assert(std::is_nothrow_copy_constructible<gsl::span<int>>::value,
               "std::is_nothrow_copy_constructible<gsl::span<int>>");

tests/span_ext_tests.cpp

@@ -320,7 +320,7 @@ TEST(span_ext_test, make_span_from_array_constructor)
      {
          int arr[] = {1, 2, 3};
 
-         span<int> s1 = {&arr[0], 2}; // shorter
+         span<int> s1 = span<int>(&arr[0], 2); // shorter
          span<int> s2 = arr;          // longer
 
          EXPECT_TRUE(s1 != s2);

tests/span_tests.cpp

@@ -157,7 +157,7 @@ TEST(span_test, from_pointer_length_constructor)
         for (int i = 0; i < 4; ++i)
         {
             {
-                span<int> s = {&arr[0], narrow_cast<std::size_t>(i)};
+                span<int> s = span<int>(&arr[0], narrow_cast<std::size_t>(i));
                 EXPECT_TRUE(s.size() == narrow_cast<std::size_t>(i));
                 EXPECT_TRUE(s.data() == &arr[0]);
                 EXPECT_TRUE(s.empty() == (i == 0));
@@ -165,7 +165,7 @@ TEST(span_test, from_pointer_length_constructor)
                     EXPECT_TRUE(arr[j] == s[narrow_cast<std::size_t>(j)]);
             }
             {
-                span<int> s = {&arr[i], 4 - narrow_cast<std::size_t>(i)};
+                span<int> s = span<int>(&arr[i], 4 - narrow_cast<std::size_t>(i));
                 EXPECT_TRUE(s.size() == 4 - narrow_cast<std::size_t>(i));
                 EXPECT_TRUE(s.data() == &arr[i]);
                 EXPECT_TRUE(s.empty() == ((4 - i) == 0));
@@ -678,7 +678,7 @@ TEST(span_test, from_array_constructor)
      s2 = s1;
      EXPECT_TRUE(s2.empty());
 
-     auto get_temp_span = [&]() -> span<int> { return {&arr[1], 2}; };
+     auto get_temp_span = [&]() -> span<int> { return span<int>(&arr[1], 2); };
      auto use_span = [&](span<const int> s) {
          EXPECT_TRUE(s.size() ==  2);
          EXPECT_TRUE(s.data() == &arr[1]);