🐛 Fix compile-time capacity calculation for dynamic span

Problem:
- `ct_capacity_v<span<int>>` returns its capacity as the maximum value of
  `std::size_t`, which is wrong.

Solution:
- Make `ct_capacity_v` a compile-time error when called on a dynamic
  `stdx::span`. (Just like on a dynamic `std::span`). These things don't have
  compile-time capacities; they are dynamic.

Author: elbeno

include/stdx/span.hpp

@@ -281,6 +281,10 @@ span(R &&) -> span<std::remove_reference_t<detail::range_reference_t<R>>>;
 
 template <typename T, std::size_t N>
 constexpr auto ct_capacity_v<span<T, N>> = N;
+
+template <typename T>
+constexpr auto ct_capacity_v<span<T, dynamic_extent>> =
+    detail::ct_capacity_fail<span<T, dynamic_extent>>{};
 } // namespace v1
 } // namespace stdx
 

test/fail/CMakeLists.txt

@@ -32,8 +32,9 @@ if(${CMAKE_CXX_STANDARD} GREATER_EQUAL 20)
         atomic_bool_dec
         call_by_need
         ct_format_mismatch
-        dynamic_span_no_ct_capacity
         dynamic_container_no_ct_capacity
+        dynamic_std_span_no_ct_capacity
+        dynamic_stdx_span_no_ct_capacity
         tuple_index_out_of_bounds
         tuple_equality_mismatch
         tuple_equality_with_element

test/fail/dynamic_span_no_ct_capacity.cpp renamed to test/fail/dynamic_std_span_no_ct_capacity.cpp

@@ -0,0 +1,13 @@
+#include <stdx/iterator.hpp>
+#include <stdx/span.hpp>
+
+#include <array>
+#include <iterator>
+
+// EXPECT: Type does not support compile-time capacity
+
+auto main() -> int {
+    auto a = std::array<int, 4>{};
+    auto s = stdx::span{std::begin(a), std::end(a)};
+    constexpr auto c = stdx::ct_capacity(s);
+}