Add checks for extent compatability in span range constructor

include/boost/core/span.hpp

@@ -153,6 +153,39 @@ struct span_bytes<T, boost::dynamic_extent> {
     static constexpr std::size_t value = boost::dynamic_extent;
 };
 
+// This is what span_default_constructed_size returns if we can not determine size at compile time,
+// or if it is of no value for check(e.g. constexpr std::vector will return 0).
+// This value can not be changed since logic only works if it is 0.
+static constexpr std::size_t span_unknown_or_useless_size = 0;
+
+// Uses better match betwen short and float argument to do second step in dispatch:
+// 1) does type has size method
+// 2) is size constexpr and does it return a usable value
+template<typename R>
+constexpr std::size_t span_default_constructed_size_dispatch(float) {
+    return span_unknown_or_useless_size;
+}
+
+template<typename R, typename std::enable_if<R().size() != span_unknown_or_useless_size, int>::type = 0>
+constexpr std::size_t span_default_constructed_size_dispatch(short) {
+    return R().size();
+}
+
+template<typename R, typename std::enable_if<span_has_size<R>::value, int>::type = 0>
+constexpr std::size_t span_default_constructed_size() {
+    return span_default_constructed_size_dispatch<typename std::remove_reference<R>::type>(short{0});
+}
+
+template<typename R, typename std::enable_if<!span_has_size<R>::value, int>::type = 0>
+constexpr std::size_t span_default_constructed_size() {
+    return span_unknown_or_useless_size;
+}
+
+template<std::size_t E, std::size_t InputE>
+constexpr bool static_extents_incompatible() {
+    return (InputE != span_unknown_or_useless_size) &&  (E != InputE);
+}
+
 } /* detail */
 
 template<class T, std::size_t E>
@@ -232,7 +265,9 @@ class span {
             detail::span_is_range<R, T>::value, int>::type = 0>
     explicit constexpr span(R&& r) noexcept(noexcept(boost::data(r)) &&
         noexcept(r.size()))
-        : s_(boost::data(r), r.size()) { }
+        : s_(boost::data(r), r.size()) {
+            static_assert(!detail::static_extents_incompatible<E, detail::span_default_constructed_size<R>()>(), "Incompatible Extents");
+         }
 
     template<class U, std::size_t N,
         typename std::enable_if<detail::span_implicit<E, N>::value &&

test/Jamfile.v2

@@ -413,6 +413,11 @@ run as_bytes_test.cpp ;
 run as_writable_bytes_test.cpp ;
 compile span_boost_begin_test.cpp ;
 run make_span_test.cpp ;
+compile-fail span_incompatible_range_size_smaller.cpp ;
+compile-fail span_incompatible_range_size_larger.cpp ;
+compile span_compatible_range_size.cpp ;
+compile span_undetectable_incompatible_range_size.cpp ;
+compile span_default_constructed_size.cpp ;
 
 run splitmix64_test.cpp
   : : : $(pedantic-errors) ;

test/span_compatible_range_size.cpp

@@ -0,0 +1,18 @@
+#include <boost/array.hpp>
+#include <boost/core/span.hpp>
+
+// use boost::array as generic range since span has no special handling for it,
+// unlike for std::array
+int main()
+{
+    boost::array<int, 0> arr0{};
+    boost::span<int, 0> sp0{arr0};
+    boost::span<const int, 0> csp0{arr0};
+    (void)sp0;
+    (void)csp0;
+    boost::array<int, 4> arr{};
+    boost::span<int, 4> sp{arr};
+    boost::span<const int, 4> csp{arr};
+    (void)sp;
+    (void)csp;
+}

test/span_default_constructed_size.cpp

@@ -0,0 +1,57 @@
+#include <boost/core/span.hpp>
+#include <boost/static_assert.hpp>
+#include <thread>
+#include <type_traits>
+#include <vector>
+#if __cplusplus >= 202302L
+#include <span>
+#endif
+using namespace boost;
+
+// well known trick to make size() nicer.
+struct MyArray3 {
+    static constexpr std::integral_constant<std::size_t, 3> size{};
+};
+
+// tests for helper used in span range constructor
+int main()
+{
+    constexpr auto vec_size = detail::span_default_constructed_size<std::vector<int>>();
+    BOOST_STATIC_ASSERT(vec_size == 0);
+    constexpr auto str_size = detail::span_default_constructed_size<std::string>();
+    BOOST_STATIC_ASSERT(str_size == 0);
+    constexpr auto arr0_size = detail::span_default_constructed_size<std::array<int, 0>>();
+    BOOST_STATIC_ASSERT(arr0_size == 0);
+    constexpr auto arr1_size = detail::span_default_constructed_size<std::array<int, 1>>();
+    BOOST_STATIC_ASSERT(arr1_size == 1);
+    constexpr auto arr2_size = detail::span_default_constructed_size<std::array<int, 2>>();
+    BOOST_STATIC_ASSERT(arr2_size == 2);
+    constexpr auto non_constexpr_element_arr_size = detail::span_default_constructed_size<std::array<std::thread, 2>>();
+    BOOST_STATIC_ASSERT(non_constexpr_element_arr_size == 0);
+
+    constexpr auto const_arr0_size = detail::span_default_constructed_size<const std::array<int, 0>>();
+    BOOST_STATIC_ASSERT(const_arr0_size == 0);
+    constexpr auto const_arr2_size = detail::span_default_constructed_size<const std::array<int, 2>>();
+    BOOST_STATIC_ASSERT(const_arr2_size == 2);
+
+#ifdef __cpp_lib_integral_constant_callable
+    constexpr auto my_array3_size = detail::span_default_constructed_size<MyArray3>();
+    BOOST_STATIC_ASSERT(my_array3_size == 3);
+#endif
+
+#if __cplusplus >= 202302L
+    // static extent span has no default constructor so we can not determine size
+    constexpr auto span0_size = detail::span_default_constructed_size<std::span<int, 0>>();
+    BOOST_STATIC_ASSERT(span0_size == 0);
+    constexpr auto span1_size = detail::span_default_constructed_size<std::span<int, 1>>();
+    BOOST_STATIC_ASSERT(span1_size == 0);
+    constexpr auto span2_size = detail::span_default_constructed_size<std::span<int, 2>>();
+    BOOST_STATIC_ASSERT(span2_size == 0);
+
+    constexpr auto const_span2_size = detail::span_default_constructed_size<std::span<const int, 2>>();
+    BOOST_STATIC_ASSERT(const_span2_size == 0);
+
+    constexpr auto dyn_span_size = detail::span_default_constructed_size<std::span<int>>();
+    BOOST_STATIC_ASSERT(dyn_span_size == 0);
+#endif
+}

test/span_incompatible_range_size_larger.cpp

@@ -0,0 +1,9 @@
+#include <boost/array.hpp>
+#include <boost/core/span.hpp>
+
+int main()
+{
+    boost::array<int, 5> arr{};
+    boost::span<int, 4> sp{arr};
+    (void)sp;
+}

test/span_incompatible_range_size_smaller.cpp

@@ -0,0 +1,9 @@
+#include <boost/array.hpp>
+#include <boost/core/span.hpp>
+
+int main()
+{
+    boost::array<int, 3> arr{};
+    boost::span<int, 4> sp{arr};
+    (void)sp;
+}

test/span_undetectable_incompatible_range_size.cpp

@@ -0,0 +1,37 @@
+#include <boost/array.hpp>
+#include <boost/core/span.hpp>
+#if __cplusplus >= 202302L
+#include <span>
+#endif
+#include <vector>
+
+int main()
+{
+    // many cases where constexpr default constructed container has 0 size, including C++20 onwards
+    // std::vector so we can not catch that case.
+    {
+        boost::array<int, 0> arr{};
+        boost::span<int, 4> sp{arr};
+        (void)sp;
+        std::vector<int> vec(10, 10);
+        boost::span<int, 4> sp2{vec};
+        (void)sp2;
+    }
+
+#if __cplusplus >= 202302L
+    // fixed extent std::span does not have default constructor so we can not get size
+    {
+        std::array<int, 4> arr{};
+        std::span<int, 4> std_sp{arr};
+        boost::span<int, 3> sp{std_sp};
+        (void)sp;
+    }
+    // no size check possible for dynamic extent std::span
+    {
+        std::array<int, 4> arr{};
+        std::span<int> std_sp{arr};
+        boost::span<int, 3> sp{std_sp};
+        (void)sp;
+    }
+#endif
+}