Merge 2021-10 LWG Motion 11

P1272R4 Byteswapping for fun&&nuf

Author: tkoeppe

source/numerics.tex

@@ -1290,6 +1290,10 @@
   template<class To, class From>
     constexpr To bit_cast(const From& from) noexcept;
 
+  // \ref{bit.byteswap}, \tcode{byteswap}
+  template<class T>
+    constexpr T byteswap(T value) noexcept;
+
   // \ref{bit.pow.two}, integral powers of 2
   template<class T>
     constexpr bool has_single_bit(T x) noexcept;
@@ -1355,6 +1359,15 @@
 if there is no value of the object's type corresponding to the
 value representation produced, the behavior is undefined.
 If there are multiple such values, which value is produced is unspecified.
+A bit in the value representation of the result is indeterminate if
+it does not correspond to a bit in the value representation of \tcode{from} or
+corresponds to a bit of an object that is not within its lifetime or
+has an indeterminate value\iref{basic.indet}.
+For each bit in the value representation of the result that is indeterminate,
+the smallest object containing that bit has an indeterminate value;
+the behavior is undefined unless that object is
+of unsigned ordinary character type or \tcode{std::byte} type.
+The result does not otherwise contain any indeterminate values.
 
 \pnum
 \remarks
@@ -1370,6 +1383,34 @@
 \end{itemize}
 \end{itemdescr}
 
+\rSec2[bit.byteswap]{\tcode{byteswap}}
+
+\indexlibraryglobal{byteswap}%
+\begin{itemdecl}
+template<class T>
+  constexpr T byteswap(T value) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\constraints
+\tcode{T} models \libconcept{integral}.
+
+\pnum
+\mandates
+\tcode{T} does not have padding bits\iref{basic.types.general}.
+
+\pnum
+Let the sequence $R$ comprise
+the bytes of the object representation of \tcode{value} in reverse order.
+
+\pnum
+\returns
+An object \tcode{v} of type \tcode{T}
+such that each byte in the object representation of \tcode{v} is equal to
+the byte in the corresponding position in $R$.
+\end{itemdescr}
+
 \rSec2[bit.pow.two]{Integral powers of 2}
 
 \indexlibraryglobal{has_single_bit}%

source/support.tex

@@ -579,6 +579,7 @@
 #define @\defnlibxname{cpp_lib_bounded_array_traits}@              201902L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_boyer_moore_searcher}@              201603L // also in \libheader{functional}
 #define @\defnlibxname{cpp_lib_byte}@                              201603L // also in \libheader{cstddef}
+#define @\defnlibxname{cpp_lib_byteswap}@                          202110L // also in \libheader{bit}
 #define @\defnlibxname{cpp_lib_char8_t}@                           201907L
   // also in \libheader{atomic}, \libheader{filesystem}, \libheader{istream}, \libheader{limits}, \libheader{locale}, \libheader{ostream}, \libheader{string}, \libheader{string_view}
 #define @\defnlibxname{cpp_lib_chrono}@                            201907L // also in \libheader{chrono}