[libcxx-commits] [PATCH] D99895: [libc++] Implement D2351R0 "Mark all library static cast wrappers as [[nodiscard]]"

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone created this revision.
Quuxplusone added reviewers: libc++, ldionne, curdeius, Mordante, cor3ntin.
Quuxplusone added a project: libc++.
Quuxplusone requested review of this revision.
Herald added a subscriber: libcxx-commits.
Herald added 1 blocking reviewer(s): libc++.

  These [[nodiscard]] annotations are added as a conforming extension;
  it's unclear whether the paper will actually be adopted and make them
  mandatory, but they do seem like good ideas regardless.
  
  https://isocpp.org/files/papers/D2351R0.pdf
  
  This patch implements the paper's effect on:
  - std::to_integer, std::to_underlying
  - std::forward, std::move, std::move_if_noexcept
  - std::as_const
  - std::identity
  
  The paper also affects (but libc++ does not yet have an implementation of):
  - std::bit_cast


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D99895

Files:
  libcxx/include/cstddef
  libcxx/include/functional
  libcxx/include/type_traits
  libcxx/include/utility


Index: libcxx/include/utility
===================================================================
--- libcxx/include/utility
+++ libcxx/include/utility
@@ -259,7 +259,7 @@
 // move_if_noexcept
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 #ifndef _LIBCPP_CXX03_LANG
 typename conditional
 <
@@ -276,8 +276,11 @@
 }
 
 #if _LIBCPP_STD_VER > 14
-template <class _Tp> constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept { return __t; }
-template <class _Tp>                        void as_const(const _Tp&&) = delete;
+template <class _Tp>
+_LIBCPP_NODISCARD_EXT constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept { return __t; }
+
+template <class _Tp>
+void as_const(const _Tp&&) = delete;
 #endif
 
 struct _LIBCPP_TEMPLATE_VIS piecewise_construct_t { explicit piecewise_construct_t() = default; };
@@ -1635,7 +1638,7 @@
 
 #if _LIBCPP_STD_VER > 20
 template <class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr underlying_type_t<_Tp>
+_LIBCPP_NODISCARD_EXT _LIBCPP_INLINE_VISIBILITY constexpr underlying_type_t<_Tp>
 to_underlying(_Tp __val) noexcept {
   return _VSTD::__to_underlying(__val);
 }
Index: libcxx/include/type_traits
===================================================================
--- libcxx/include/type_traits
+++ libcxx/include/type_traits
@@ -2784,7 +2784,7 @@
 // move
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 typename remove_reference<_Tp>::type&&
 move(_Tp&& __t) _NOEXCEPT
 {
@@ -2793,7 +2793,7 @@
 }
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Tp&&
 forward(typename remove_reference<_Tp>::type& __t) _NOEXCEPT
 {
@@ -2801,12 +2801,12 @@
 }
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Tp&&
 forward(typename remove_reference<_Tp>::type&& __t) _NOEXCEPT
 {
     static_assert(!is_lvalue_reference<_Tp>::value,
-                  "can not forward an rvalue as an lvalue");
+                  "cannot forward an rvalue as an lvalue");
     return static_cast<_Tp&&>(__t);
 }
 
Index: libcxx/include/functional
===================================================================
--- libcxx/include/functional
+++ libcxx/include/functional
@@ -3221,7 +3221,7 @@
 // [func.identity]
 struct identity {
     template<class _Tp>
-    constexpr _Tp&& operator()(_Tp&& __t) const noexcept
+    _LIBCPP_NODISCARD_EXT constexpr _Tp&& operator()(_Tp&& __t) const noexcept
     {
         return _VSTD::forward<_Tp>(__t);
     }
Index: libcxx/include/cstddef
===================================================================
--- libcxx/include/cstddef
+++ libcxx/include/cstddef
@@ -152,7 +152,7 @@
   { return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(__lhs) >> __shift)); }
 
 template <class _Integer, class = _EnableByteOverload<_Integer> >
-  constexpr _Integer
+  _LIBCPP_NODISCARD_EXT constexpr _Integer
   to_integer(byte __b) noexcept { return static_cast<_Integer>(__b); }
 }
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D99895.335307.patch
Type: text/x-patch
Size: 3293 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/libcxx-commits/attachments/20210405/0ea1c1b7/attachment.bin>