[libcxx] span: Fix incorrect return type of span::subspan

[miscco]

Summary

The extent of the returned span was incorrect

[miscco]

Gentle ping with the relevant quote from the standard

[miscco]

Ping as this is an actual bugfix

[ldionne]

Thanks for the patch! Good find. I'll commit this with a slight change to the tests. I'll use this formulation of the static_asserts instead, which is clearer IMO:

if constexpr (Count != std::dynamic_extent) {
    static_assert(S1::extent == Count);
} else if constexpr (Span::extent != std::dynamic_extent) {
    static_assert(S1::extent == Span::extent - Offset);
} else {
    static_assert(S1::extent == std::dynamic_extent);
}

libcxx/include/span

//    ~span() noexcept = default;
     constexpr span<element_type, dynamic_extent> last (size_type __count) const noexcept
     {
         _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
         return {data() + size() - __count, __count};
     }
 
     template <size_t _Offset, size_t _Count = dynamic_extent>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span<_Tp, dynamic_extent> subspan() const noexcept
+    constexpr span<element_type, _Count> subspan() const noexcept
     {
         _LIBCPP_ASSERT(_Offset <= size(), "Offset out of range in span::subspan()");
         _LIBCPP_ASSERT(_Count == dynamic_extent || _Offset + _Count <= size(), "Count out of range in span::subspan()");
         return {data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
     }
 
     constexpr span<element_type, dynamic_extent>
     _LIBCPP_INLINE_VISIBILITY

libcxx/test/std/containers/views/span.sub/subspan.pass.cpp

constexpr bool testConstexprSpan(Span sp)
     LIBCPP_ASSERT((noexcept(sp.template subspan<Offset, Count>())));
     LIBCPP_ASSERT((noexcept(sp.subspan(Offset, Count))));
     auto s1 = sp.template subspan<Offset, Count>();
     auto s2 = sp.subspan(Offset, Count);
     using S1 = decltype(s1);
     using S2 = decltype(s2);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S1::value_type);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S2::value_type);
-    static_assert(S1::extent == (Span::extent == std::dynamic_extent ? std::dynamic_extent : Count), "");
-    static_assert(S2::extent == std::dynamic_extent, "");
+    static_assert(S1::extent ==
+      ((Count != std::dynamic_extent) ? Count
+                      : ((Span::extent != std::dynamic_extent) ? Span::extent - Offset
+                                           : std::dynamic_extent)), "");
+  static_assert(S2::extent == std::dynamic_extent, "");
     return
         s1.data() == s2.data()
      && s1.size() == s2.size()
      && std::equal(s1.begin(), s1.end(), sp.begin() + Offset);
 }
 
 template <typename Span, size_t Offset>
 constexpr bool testConstexprSpan(Span sp)
void testRuntimeSpan(Span sp)
     LIBCPP_ASSERT((noexcept(sp.template subspan<Offset, Count>())));
     LIBCPP_ASSERT((noexcept(sp.subspan(Offset, Count))));
     auto s1 = sp.template subspan<Offset, Count>();
     auto s2 = sp.subspan(Offset, Count);
     using S1 = decltype(s1);
     using S2 = decltype(s2);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S1::value_type);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S2::value_type);
-    static_assert(S1::extent == (Span::extent == std::dynamic_extent ? std::dynamic_extent : Count), "");
+    static_assert(S1::extent ==
+      ((Count != std::dynamic_extent) ? Count
+                      : ((Span::extent != std::dynamic_extent) ? Span::extent - Offset
+                                           : std::dynamic_extent)), "");
     static_assert(S2::extent == std::dynamic_extent, "");
     assert(s1.data() == s2.data());
     assert(s1.size() == s2.size());
     assert(std::equal(s1.begin(), s1.end(), sp.begin() + Offset));
 }
 
 
 template <typename Span, size_t Offset>