[libcxx][span] Implement solution to LWG-3255

[miscco]

Summary

This implements the relaxed requirements on the std::array constructors of span,
where the type only needs to be convertible to the element type of the span.

Note that the previous tests were not sufficient, as the const array<T, n> constructor
was only tested for compile time and the array<T, N> only during runtime.

Restructure the tests so that we can test conversions as well as both constructors.

[miscco]

ping

[miscco]

ping

[miscco]

Thanks for the review, could somebody commit it as I do not have the rights

libcxx/include/span

// [span.cons], span constructors, copy, assignment, and destructor
     constexpr span& operator=(const span&) noexcept = default;
 
     _LIBCPP_INLINE_VISIBILITY constexpr explicit span(pointer __ptr, size_type __count) : __data{__ptr}
         { (void)__count; _LIBCPP_ASSERT(_Extent == __count, "size mismatch in span's constructor (ptr, len)"); }
     _LIBCPP_INLINE_VISIBILITY constexpr explicit span(pointer __f, pointer __l) : __data{__f}
         { (void)__l;     _LIBCPP_ASSERT(_Extent == distance(__f, __l), "size mismatch in span's constructor (ptr, ptr)"); }
 
     _LIBCPP_INLINE_VISIBILITY constexpr span(element_type (&__arr)[_Extent])          noexcept : __data{__arr} {}
-    _LIBCPP_INLINE_VISIBILITY constexpr span(      array<value_type, _Extent>& __arr) noexcept : __data{__arr.data()} {}
-    _LIBCPP_INLINE_VISIBILITY constexpr span(const array<value_type, _Extent>& __arr) noexcept : __data{__arr.data()} {}
+
+    template <class _OtherElementType,
+              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
+
+    template <class _OtherElementType,
+              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
 
     template <class _Container>
     _LIBCPP_INLINE_VISIBILITY
         constexpr explicit span(      _Container& __c,
             enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
         : __data{_VSTD::data(__c)} {
             _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (range)");
         }
// [span.cons], span constructors, copy, assignment, and destructor
 
     _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __ptr, size_type __count) : __data{__ptr}, __size{__count} {}
     _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __f, pointer __l) : __data{__f}, __size{static_cast<size_t>(distance(__f, __l))} {}
 
     template <size_t _Sz>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(element_type (&__arr)[_Sz])          noexcept : __data{__arr}, __size{_Sz} {}
 
-    template <size_t _Sz>
+    template <class _OtherElementType, size_t _Sz,
+              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(array<value_type, _Sz>& __arr)       noexcept : __data{__arr.data()}, __size{_Sz} {}
+    constexpr span(array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
 
-    template <size_t _Sz>
+    template <class _OtherElementType, size_t _Sz,
+              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(const array<value_type, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
+    constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
 
     template <class _Container>
     _LIBCPP_INLINE_VISIBILITY
         constexpr span(      _Container& __c,
             enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
         : __data{_VSTD::data(__c)}, __size{(size_type) _VSTD::size(__c)} {}
 
     template <class _Container>

libcxx/test/std/containers/views/span.cons/stdarray.pass.cpp

//  types different (static sized)
     {
     std::span<const          int,3> s1{ arr};   // a span<const          int> pointing at int.
     std::span<      volatile int,3> s2{ arr};   // a span<      volatile int> pointing at int.
     std::span<      volatile int,3> s3{ arr};   // a span<      volatile int> pointing at const int.
     std::span<const volatile int,3> s4{ arr};   // a span<const volatile int> pointing at int.
     }
 }
 
+template <typename T, typename U = T>
+constexpr bool testConstructorArray()
+{
+    std::array<U,2> val = { U(), U() };
+    ASSERT_NOEXCEPT(std::span<T>   {val});
+    ASSERT_NOEXCEPT(std::span<T, 2>{val});
+    std::span<T>    s1{val};
+    std::span<T, 2> s2{val};
+    return s1.data() == &val[0] && s1.size() == 2
+        && s2.data() == &val[0] && s2.size() == 2;
+}
 
-template <typename T>
-constexpr bool testConstexprSpan()
+template <typename T, typename U = T>
+constexpr bool testConstructorConstArray()
 {
-    constexpr std::array<T,2> val = { T(), T() };
+    const std::array<U,2> val = { U(), U() };
     ASSERT_NOEXCEPT(std::span<const T>   {val});
     ASSERT_NOEXCEPT(std::span<const T, 2>{val});
     std::span<const T>    s1{val};
     std::span<const T, 2> s2{val};
-    return
-        s1.data() == &val[0] && s1.size() == 2
-    &&  s2.data() == &val[0] && s2.size() == 2;
+    return s1.data() == &val[0] && s1.size() == 2
+        && s2.data() == &val[0] && s2.size() == 2;
 }
 
-
 template <typename T>
-void testRuntimeSpan()
-{
-    std::array<T,2> val;
-    ASSERT_NOEXCEPT(std::span<T>   {val});
-    ASSERT_NOEXCEPT(std::span<T, 2>{val});
-    std::span<T>    s1{val};
-    std::span<T, 2> s2{val};
-    assert(s1.data() == &val[0] && s1.size() == 2);
-    assert(s2.data() == &val[0] && s2.size() == 2);
+constexpr bool testConstructors() {
+    static_assert(testConstructorArray<T>(), "");
+    static_assert(testConstructorArray<const T, T>(), "");
+    static_assert(testConstructorConstArray<T>(), "");
+    static_assert(testConstructorConstArray<const T, T>(), "");
+
+    return testConstructorArray<T>()
+        && testConstructorArray<const T, T>()
+        && testConstructorConstArray<T>()
+        && testConstructorConstArray<const T, T>();
 }
 
 struct A{};
 
 int main(int, char**)
 {
-    static_assert(testConstexprSpan<int>(),    "");
-    static_assert(testConstexprSpan<long>(),   "");
-    static_assert(testConstexprSpan<double>(), "");
-    static_assert(testConstexprSpan<A>(),      "");
-
-    testRuntimeSpan<int>();
-    testRuntimeSpan<long>();
-    testRuntimeSpan<double>();
-    testRuntimeSpan<std::string>();
-    testRuntimeSpan<A>();
+    assert(testConstructors<int>());
+    assert(testConstructors<long>());
+    assert(testConstructors<double>());
+    assert(testConstructors<A>());
+
+    assert(testConstructors<int*>());
+    assert(testConstructors<int* const>());
+    assert(testConstructors<const int*>());
+    assert(testConstructors<const int* const>());
 
     checkCV();
 
     return 0;
 }

libcxx/www/cxx2a_status.html

-->
   <tr><td><a href="https://wg21.link/LWG3242">3242</a></td><td><tt>std::format</tt>: missing rules for <tt>arg-id</tt> in <tt>width</tt> and <tt>precision</tt></td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3243">3243</a></td><td><tt>std::format</tt> and negative zeroes</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3247">3247</a></td><td><tt>ranges::iter_move</tt> should perform ADL-only lookup of <tt>iter_move</tt></td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3248">3248</a></td><td><tt>std::format</tt> <tt>#b</tt>, <tt>#B</tt>, <tt>#o</tt>, <tt>#x</tt>, and <tt>#X</tt>  presentation types misformat negative numbers</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3250">3250</a></td><td><tt>std::format</tt>: <tt>#</tt> (alternate form) for NaN and inf</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3251">3251</a></td><td>Are <tt>std::format</tt> alignment specifiers applied to string arguments?</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3252">3252</a></td><td>Parse locale's aware modifiers for commands are not consistent with POSIX spec</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3254">3254</a></td><td>Strike <tt>stop_token</tt>'s <tt>operator!=</tt></td><td>Prague</td><td></td></tr>
-  <tr><td><a href="https://wg21.link/LWG3255">3255</a></td><td><tt>span</tt>'s <tt>array</tt> constructor is too strict</td><td>Prague</td><td></td></tr>
+  <tr><td><a href="https://wg21.link/LWG3255">3255</a></td><td><tt>span</tt>'s <tt>array</tt> constructor is too strict</td><td>Prague</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG3260">3260</a></td><td><tt>year_month*</tt> arithmetic rejects durations convertible to years</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3262">3262</a></td><td>Formatting of negative durations is not specified</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3264">3264</a></td><td><tt>sized_range</tt> and <tt>ranges::size</tt> redundantly use <tt>disable_sized_range</tt></td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3269">3269</a></td><td>Parse manipulators do not specify the result of the extraction from stream</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3270">3270</a></td><td>Parsing and formatting <tt>%j</tt> with <tt>duration</tt>s</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3280">3280</a></td><td>View converting constructors can cause constraint recursion and are unneeded</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3281">3281</a></td><td>Conversion from <tt><i>pair-like</i></tt> types to <tt>subrange</tt> is a silent semantic promotion</td><td>Prague</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG3282">3282</a></td><td><tt>subrange</tt> converting constructor should disallow derived to base conversions</td><td>Prague</td><td></td></tr>