commit 406f58e1e38e92e4b881f3666b596843da308783
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Apr 6 14:41:29 2021

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

diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h
index feacae084c9..3abbb37ceeb 100644
--- a/libstdc++-v3/include/bits/move.h
+++ b/libstdc++-v3/include/bits/move.h
@@ -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); }
diff --git a/libstdc++-v3/include/bits/ranges_cmp.h b/libstdc++-v3/include/bits/ranges_cmp.h
index 3f71d31e5a6..f859a33b2c1 100644
--- a/libstdc++-v3/include/bits/ranges_cmp.h
+++ b/libstdc++-v3/include/bits/ranges_cmp.h
@@ -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); }
diff --git a/libstdc++-v3/include/c_global/cstddef b/libstdc++-v3/include/c_global/cstddef
index 11c808cab28..13ef7f03c12 100644
--- a/libstdc++-v3/include/c_global/cstddef
+++ b/libstdc++-v3/include/c_global/cstddef
@@ -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); }
diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit
index 8638a02c8a6..fb78578448c 100644
--- a/libstdc++-v3/include/std/bit
+++ b/libstdc++-v3/include/std/bit
@@ -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
     {
diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index fb19d62968f..3e68f682e00 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -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); }