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[2025-11 LWG Motion 14] P1789R3 Library Support for Expansion Statements #8482

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[2025-11 LWG Motion 14] P1789R3 Library Support for Expansion Statements #8482

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P1789R3 Library Support for Expansion Statements
Tsche Nov 9, 2025
01a2ba0
Fixup: resolve minor formatting issues, fixup comment in [utility.syn]
Tsche Nov 10, 2025
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41 changes: 41 additions & 0 deletions source/meta.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,47 @@
\end{note}
\end{itemdescr}

\rSec2[intseq.binding]{Structured binding support}

\indexlibraryglobal{tuple_size}%
\indexlibraryglobal{tuple_element}%
\begin{itemdecl}
template<class T, T... Values>
struct tuple_size<integer_sequence<T, Values...>>
: integral_constant<size_t, sizeof...(Values)> { };

template<size_t I, class T, T... Values>
struct tuple_element<I, integer_sequence<T, Values...>> {
using type = T;
};
template<size_t I, class T, T... Values>
struct tuple_element<I, const integer_sequence<T, Values...>> {
using type = T;
};
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@eisenwave eisenwave Nov 9, 2025

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We don't normally apply two-space indentation to definitions of classes. However, we're not consistent about that in general. There are definitions like struct nontype_t which are indented.

I don't think it needs to be fixed here, just something to keep in mind.

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@Tsche Tsche Nov 10, 2025

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We seem to sometimes do that for short class definitions and empty classes (which would be the case for tuple_size directly above).

Currently tuple_element specializations for array, tuple and pair use 2-space indentation for the definition. Specializations for complex and ranges::subrange do not.

I've left it as is for now.

\end{itemdecl}

\begin{itemdescr}
\pnum
\mandates
$\tcode{I} < \tcode{sizeof...(Values)}$.
\end{itemdescr}

\indexlibrarymember{get}{integer_sequence}%
\begin{itemdecl}
template<size_t I, class T, T... Values>
constexpr T get(integer_sequence<T, Values...>) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\mandates
$\tcode{I} < \tcode{sizeof...(Values)}$.

\pnum
\returns
\tcode{Values...[I]}.
\end{itemdescr}

\rSec1[type.traits]{Metaprogramming and type traits}

\rSec2[type.traits.general]{General}
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2 changes: 1 addition & 1 deletion source/support.tex
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Expand Up @@ -725,7 +725,7 @@
#define @\defnlibxname{cpp_lib_inplace_vector}@ 202406L // also in \libheader{inplace_vector}
#define @\defnlibxname{cpp_lib_int_pow2}@ 202002L // freestanding, also in \libheader{bit}
#define @\defnlibxname{cpp_lib_integer_comparison_functions}@ 202002L // also in \libheader{utility}
#define @\defnlibxname{cpp_lib_integer_sequence}@ 201304L // freestanding, also in \libheader{utility}
#define @\defnlibxname{cpp_lib_integer_sequence}@ 202511L // freestanding, also in \libheader{utility}
#define @\defnlibxname{cpp_lib_integral_constant_callable}@ 201304L // freestanding, also in \libheader{type_traits}
#define @\defnlibxname{cpp_lib_interpolate}@ 201902L // also in \libheader{cmath}, \libheader{numeric}
#define @\defnlibxname{cpp_lib_invoke}@ 201411L // freestanding, also in \libheader{functional}
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18 changes: 15 additions & 3 deletions source/utilities.tex
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Expand Up @@ -114,6 +114,21 @@
template<class... T>
using index_sequence_for = make_index_sequence<sizeof...(T)>;

template<class T> struct tuple_size;
template<size_t I, class T> struct tuple_element;

// \ref{intseq.binding}, structured binding support
template<class T, T... Values>
struct tuple_size<integer_sequence<T, Values...>>;

template<size_t I, class T, T... Values>
struct tuple_element<I, integer_sequence<T, Values...>>;
template<size_t I, class T, T... Values>
struct tuple_element<I, const integer_sequence<T, Values...>>;

template<size_t I, class T, T... Values>
constexpr T get(integer_sequence<T, Values...>) noexcept;

// \ref{pairs}, class template \tcode{pair}
template<class T1, class T2>
struct pair;
Expand Down Expand Up @@ -151,9 +166,6 @@
constexpr @\seebelow@ make_pair(T1&&, T2&&);

// \ref{pair.astuple}, tuple-like access to pair
template<class T> struct tuple_size;
template<size_t I, class T> struct tuple_element;

template<class T1, class T2> struct tuple_size<pair<T1, T2>>;
template<size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;

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