[libcxx-commits] [PATCH] D71994: SFINAE span default constructor on Extent == 0

[Michael Schellenberger Costa via Phabricator via libcxx-commits]

miscco created this revision.
Herald added subscribers: libcxx-commits, christof.
Herald added a project: libc++.

The default constructor of a static span requires _Extent == 0 so SFINAE it out rather than using a static_assert


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D71994

Files:
  libcxx/include/span
  libcxx/test/std/containers/views/span.cons/default.fail.cpp


Index: libcxx/test/std/containers/views/span.cons/default.fail.cpp
===================================================================
--- libcxx/test/std/containers/views/span.cons/default.fail.cpp
+++ libcxx/test/std/containers/views/span.cons/default.fail.cpp
@@ -13,7 +13,7 @@
 // constexpr span() noexcept;
 //
 //  Remarks: This constructor shall not participate in overload resolution
-//          unless Extent <= 0 is true.
+//          unless Extent == 0 || Extent == dynamic_extent is true.
 
 
 #include <span>
@@ -24,10 +24,7 @@
 
 int main(int, char**)
 {
-    std::span<int, 2> s; // expected-error-re at span:* {{static_assert failed{{( due to requirement '.*')?}} "Can't default construct a statically sized span with size > 0"}}
-
-//  TODO: This is what I want:
-// eXpected-error {{no matching constructor for initialization of 'std::span<int, 2>'}}
+  std::span<int, 2> s; // expected-error {{no matching constructor for initialization of 'std::span<int, 2>'}}
 
   return 0;
 }
Index: libcxx/include/span
===================================================================
--- libcxx/include/span
+++ libcxx/include/span
@@ -209,8 +209,8 @@
     static constexpr size_type extent = _Extent;
 
 // [span.cons], span constructors, copy, assignment, and destructor
-    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}
-    { static_assert(_Extent == 0, "Can't default construct a statically sized span with size > 0"); }
+    template <size_t _Sz = _Extent, enable_if_t<_Sz == 0, nullptr_t> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr} {}
 
     constexpr span           (const span&) noexcept = default;
     constexpr span& operator=(const span&) noexcept = default;


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