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libstdc++: Add nodiscard attribute to cast-like functions

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Add [[nodiscard]] to functions that are effectively just a static_cast,
as per P2351. Also add it to std::addressof.

libstdc++-v3/ChangeLog:

	* include/bits/move.h (forward, move, move_if_noexcept)
	(addressof): Add _GLIBCXX_NODISCARD.
	* include/bits/ranges_cmp.h (identity::operator()): Add
	nodiscard attribute.
	* include/c_global/cstddef (to_integer): Likewise.
	* include/std/bit (bit_cast): Likewise.
	* include/std/utility (as_const, to_underlying): Likewise.
This commit is contained in:
Jonathan Wakely 2021-04-06 14:41:29 +01:00
parent 41019bfae2
commit 406f58e1e3
5 changed files with 13 additions and 1 deletions
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5
libstdc++-v3/include/bits/move.h
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@ -72,6 +72,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* This function is used to implement "perfect forwarding".
*/
template<typename _Tp>
_GLIBCXX_NODISCARD
constexpr _Tp&&
forward(typename std::remove_reference<_Tp>::type& __t) noexcept
{ return static_cast<_Tp&&>(__t); }
@ -83,6 +84,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* This function is used to implement "perfect forwarding".
*/
template<typename _Tp>
_GLIBCXX_NODISCARD
constexpr _Tp&&
forward(typename std::remove_reference<_Tp>::type&& __t) noexcept
{
@ -97,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* @return The parameter cast to an rvalue-reference to allow moving it.
*/
template<typename _Tp>
_GLIBCXX_NODISCARD
constexpr typename std::remove_reference<_Tp>::type&&
move(_Tp&& __t) noexcept
{ return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
@ -116,6 +119,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* type is copyable, in which case an lvalue-reference is returned instead.
*/
template<typename _Tp>
_GLIBCXX_NODISCARD
constexpr typename
conditional<__move_if_noexcept_cond<_Tp>::value, const _Tp&, _Tp&&>::type
move_if_noexcept(_Tp& __x) noexcept
@ -136,6 +140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* @return The actual address.
*/
template<typename _Tp>
_GLIBCXX_NODISCARD
inline _GLIBCXX17_CONSTEXPR _Tp*
addressof(_Tp& __r) noexcept
{ return std::__addressof(__r); }

1
libstdc++-v3/include/bits/ranges_cmp.h
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@ -47,6 +47,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
struct identity
{
template<typename _Tp>
[[nodiscard]]
constexpr _Tp&&
operator()(_Tp&& __t) const noexcept
{ return std::forward<_Tp>(__t); }

1
libstdc++-v3/include/c_global/cstddef
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@ -169,6 +169,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return __l = __l ^ __r; }
template<typename _IntegerType>
[[nodiscard]]
constexpr _IntegerType
to_integer(__byte_op_t<_IntegerType> __b) noexcept
{ return _IntegerType(__b); }

1
libstdc++-v3/include/std/bit
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@ -54,6 +54,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/// Create a value of type `To` from the bits of `from`.
template<typename _To, typename _From>
[[nodiscard]]
constexpr _To
bit_cast(const _From& __from) noexcept
{

6
libstdc++-v3/include/std/utility
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@ -386,7 +386,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#define __cpp_lib_as_const 201510
template<typename _Tp>
constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept { return __t; }
[[nodiscard]]
constexpr add_const_t<_Tp>&
as_const(_Tp& __t) noexcept
{ return __t; }
template<typename _Tp>
void as_const(const _Tp&&) = delete;
@ -466,6 +469,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#define __cpp_lib_to_underlying 202102L
/// Convert an object of enumeration type to its underlying type.
template<typename _Tp>
[[nodiscard]]
constexpr underlying_type_t<_Tp>
to_underlying(_Tp __value) noexcept
{ return static_cast<underlying_type_t<_Tp>>(__value); }