Added conversion ctors.

include/span.h

@@ -92,6 +92,53 @@ template <class T>
 struct is_span : is_span_oracle<std::remove_cv_t<T>>
 {
 };
+
+template <class From, class To>
+struct is_allowed_pointer_conversion
+    : std::integral_constant<bool,
+        std::is_pointer<From>::value &&
+        std::is_pointer<To>::value &&
+        std::is_convertible<From, To>::value
+    >
+{
+};
+
+template <class From, class To>
+struct is_allowed_integral_conversion
+    : std::integral_constant<bool,
+        std::is_integral<From>::value &&
+        std::is_integral<To>::value &&
+        sizeof(From) == sizeof(To) && 
+        alignof(From) == alignof(To) &&
+        std::is_convertible<From, To>::value
+    >
+{
+};
+
+template <class From, class To>
+struct is_allowed_element_type_conversion
+    : std::integral_constant<bool,        
+        std::is_same<From, std::remove_cv_t<To>>::value ||
+        is_allowed_pointer_conversion<From, To>::value ||
+        is_allowed_integral_conversion<From, To>::value
+    >
+{
+};
+
+template <class From>
+struct is_allowed_element_type_conversion<From, char>
+    : std::integral_constant<bool, !std::is_const<From>::value>
+{
+};
+
+template <class From>
+struct is_allowed_element_type_conversion<From, const char>
+    : std::integral_constant<bool, true>
+{
+};
+
+
+
 } // namespace details
 
 
@@ -163,13 +210,29 @@ public:
     >
     span(const Container&&) = delete;
     
-#if 0 // TODO
     constexpr span(const span& other) noexcept = default;
     constexpr span(span&& other) noexcept = default;
-    template <class OtherElementType, ptrdiff_t OtherExtent>
+
-    constexpr span(const span<OtherElementType, OtherExtent>& other);
+    template <class OtherElementType, std::ptrdiff_t OtherExtent,
-    template <class OtherElementType, ptrdiff_t OtherExtent>
+        class = std::enable_if_t<!std::is_same<element_type, OtherElementType>::value &&
-    constexpr span(span<OtherElementType, OtherExtent>&& other);
+            details::is_allowed_element_type_conversion<OtherElementType, element_type>::value
+        >
+    >
+    constexpr span(const span<OtherElementType, OtherExtent>& other)
+        : storage_(reinterpret_cast<pointer>(other.data()), other.length())
+    {}
+
+    template <class OtherElementType, std::ptrdiff_t OtherExtent,
+        class = std::enable_if_t<!std::is_same<element_type, OtherElementType>::value &&
+            details::is_allowed_element_type_conversion<OtherElementType, element_type>::value
+        >
+    >
+    constexpr span(span<OtherElementType, OtherExtent>&& other)
+        : storage_(reinterpret_cast<pointer>(other.data()), other.length())
+    {
+    }
+
+#if 0 // TODO
     ~span() noexcept = default;
     constexpr span& operator=(const span& other) noexcept = default;
     constexpr span& operator=(span&& other) noexcept = default;

tests/span_tests.cpp

@@ -512,34 +512,44 @@ SUITE(span_tests)
 #endif
         }
     }
-#if 0
+
     TEST(from_convertible_span_constructor)
     {
+        {
+            span<DerivedClass> avd;
+            span<const DerivedClass> avcd = avd;
+            (void)avcd;
+        }
+
+        {
 #ifdef CONFIRM_COMPILATION_ERRORS
-        span<int, 7, 4, 2> av1(nullptr, b1);
+            span<DerivedClass> avd;
-
+            span<BaseClass> avb = avd;
-        auto f = [&]() { span<int, 7, 4, 2> av1(nullptr); };
+            (void) avb;
-        CHECK_THROW(f(), fail_fast);
 #endif
+        }
 
+        {
+            span<int> s;
+            span<unsigned int> s2 = s;
+            (void)s2;
+        }
+
+        {
+            span<int> s;
+            span<const unsigned int> s2 = s;
+            (void)s2;
+        }
+
+        {
 #ifdef CONFIRM_COMPILATION_ERRORS
-        static_bounds<size_t, 7, dynamic_range, 2> b12(b11);
+            span<int> s;
-        b12 = b11;
+            span<short> s2 = s;
-        b11 = b12;
+            (void)s2;
-
-        span<int, dynamic_range> av1 = nullptr;
-        span<int, 7, dynamic_range, 2> av2(av1);
-        span<int, 7, 4, 2> av2(av1);
 #endif
-
+        }
-        span<DerivedClass> avd;
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<BaseClass> avb = avd;
-#endif
-        span<const DerivedClass> avcd = avd;
-        (void) avcd;
     }
-
+#if 0
     TEST(copy_move_and_assignment)
     {
         span<int> s1;