[libc++] span: Fix incorrect return type of span::subspan

The extent of the returned span was always std::dynamic_extent, which
is incorrect.

Thanks to Michael Schellenberger Costa for the patch.

Differential Revision: https://reviews.llvm.org/D71997

diff --git a/libcxx/include/span b/libcxx/include/span
index f18d00c..a5362ca 100644 (file)
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -444,7 +444,7 @@ public:
 
     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()");

diff --git a/libcxx/test/std/containers/views/span.sub/subspan.pass.cpp b/libcxx/test/std/containers/views/span.sub/subspan.pass.cpp
index caa7b56..e46dcc1 100644 (file)
--- a/libcxx/test/std/containers/views/span.sub/subspan.pass.cpp
+++ b/libcxx/test/std/containers/views/span.sub/subspan.pass.cpp
@@ -37,7 +37,13 @@ constexpr bool testConstexprSpan(Span sp)
     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), "");
+    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);
+    }
     static_assert(S2::extent == std::dynamic_extent, "");
     return
         s1.data() == s2.data()
@@ -76,7 +82,13 @@ void testRuntimeSpan(Span sp)
     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), "");
+    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);
+    }
     static_assert(S2::extent == std::dynamic_extent, "");
     assert(s1.data() == s2.data());
     assert(s1.size() == s2.size());