From 1247cb3ed88c415ced10f680fb5b8a54b4cfeed7 Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Thu, 23 Oct 2025 12:13:25 -0600
Subject: [PATCH] :bug: 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.
---
 include/stdx/span.hpp                               |  4 ++++
 test/fail/CMakeLists.txt                            |  3 ++-
 ...city.cpp => dynamic_std_span_no_ct_capacity.cpp} |  0
 test/fail/dynamic_stdx_span_no_ct_capacity.cpp      | 13 +++++++++++++
 4 files changed, 19 insertions(+), 1 deletion(-)
 rename test/fail/{dynamic_span_no_ct_capacity.cpp => dynamic_std_span_no_ct_capacity.cpp} (100%)
 create mode 100644 test/fail/dynamic_stdx_span_no_ct_capacity.cpp

diff --git a/include/stdx/span.hpp b/include/stdx/span.hpp
index 49d5cfd..87c9f18 100644
--- a/include/stdx/span.hpp
+++ b/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
 
diff --git a/test/fail/CMakeLists.txt b/test/fail/CMakeLists.txt
index 64ccc93..09edb36 100644
--- a/test/fail/CMakeLists.txt
+++ b/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
diff --git a/test/fail/dynamic_span_no_ct_capacity.cpp b/test/fail/dynamic_std_span_no_ct_capacity.cpp
similarity index 100%
rename from test/fail/dynamic_span_no_ct_capacity.cpp
rename to test/fail/dynamic_std_span_no_ct_capacity.cpp
diff --git a/test/fail/dynamic_stdx_span_no_ct_capacity.cpp b/test/fail/dynamic_stdx_span_no_ct_capacity.cpp
new file mode 100644
index 0000000..6c01b7e
--- /dev/null
+++ b/test/fail/dynamic_stdx_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);
+}