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[2026-03 LWG Motion 16] P3948R1 constant_wrapper is the only tool needed #8878

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[2026-03 LWG Motion 16] P3948R1 constant_wrapper is the only tool needed #8878

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ef1752d
P3948R1 constant_wrapper is the only tool needed for passing constant…
eisenwave Mar 28, 2026
e4dda2f
[func.wrap.ref.deduct] Replace constant_arg_t with constant_wrapper i…
eisenwave Apr 1, 2026
6c248b5
Fixup: typo
eisenwave Apr 1, 2026
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78 changes: 35 additions & 43 deletions source/utilities.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -225,15 +225,6 @@
};
template<size_t I> constexpr in_place_index_t<I> in_place_index{};

// \tcode{constant_arg} argument tag%
\indexlibraryglobal{constant_arg_t}%
\indexlibraryglobal{constant_arg}
template<auto V>
struct constant_arg_t {
explicit constant_arg_t() = default;
};
template<auto V> constexpr constant_arg_t<V> constant_arg{};

// \ref{variant.monostate}, class \tcode{monostate}%
\indexlibraryglobal{monostate}
struct monostate;
Expand Down Expand Up @@ -15178,9 +15169,11 @@
// \ref{func.wrap.ref.ctor}, constructors and assignment operators
template<class F> function_ref(F*) noexcept;
template<class F> constexpr function_ref(F&&) noexcept;
template<auto f> constexpr function_ref(constant_arg_t<f>) noexcept;
template<auto f, class U> constexpr function_ref(constant_arg_t<f>, U&&) noexcept;
template<auto f, class T> constexpr function_ref(constant_arg_t<f>, @\cv{}@ T*) noexcept;
template<auto c, class F> constexpr function_ref(constant_wrapper<c, F>) noexcept;
template<auto c, class F, class U>
constexpr function_ref(constant_wrapper<c, F>, U&&) noexcept;
template<auto c, class F, class T>
constexpr function_ref(constant_wrapper<c, F>, @\cv{}@ T*) noexcept;

constexpr function_ref(const function_ref&) noexcept = default;
constexpr function_ref& operator=(const function_ref&) noexcept = default;
Expand All @@ -15200,10 +15193,10 @@
// \ref{func.wrap.ref.deduct}, deduction guides
template<class F>
function_ref(F*) -> function_ref<F>;
template<auto f>
function_ref(constant_arg_t<f>) -> function_ref<@\seebelow@>;
template<auto f, class T>
function_ref(constant_arg_t<f>, T&&) -> function_ref<@\seebelow@>;
template<auto c, class F0>
function_ref(constant_wrapper<c, F0>) -> function_ref<@\seebelow@>;
template<auto c, class F, class T>
function_ref(constant_wrapper<c, F>, T&&) -> function_ref<@\seebelow@>;
}
\end{codeblock}

Expand Down Expand Up @@ -15311,21 +15304,27 @@

\indexlibraryctor{function_ref}%
\begin{itemdecl}
template<auto f> constexpr function_ref(constant_arg_t<f>) noexcept;
template<auto c, class F> constexpr function_ref(constant_wrapper<c, F> f) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
Let \tcode{F} be \tcode{decltype(f)}.

\pnum
\constraints
\tcode{\exposid{is-invocable-using}<const F\&>} is \tcode{true}.

\pnum
\mandates
\begin{itemize}
\item
If \tcode{is_pointer_v<F> || is_member_pointer_v<F>} is \tcode{true},
then \tcode{f != nullptr} is \tcode{true}.
then \tcode{f.value != nullptr} is \tcode{true}, and
\item
if \tcode{ArgTypes} is not an empty pack and
all types in \tcode{remove_cvref_t<ArgTypes>...}
satisfy \exposconcept{constexpr-param} then
\tcode{constant_wrapper<\placeholdernc{INVOKE}(f.value, remove_cvref_t<ArgTypes>::\newline value...)>}
is not a valid type.
\end{itemize}

\pnum
\effects
Expand All @@ -15341,14 +15340,13 @@

\indexlibraryctor{function_ref}%
\begin{itemdecl}
template<auto f, class U>
constexpr function_ref(constant_arg_t<f>, U&& obj) noexcept;
template<auto c, class F, class U>
constexpr function_ref(constant_wrapper<c, F> f, U&& obj) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
Let \tcode{T} be \tcode{remove_reference_t<U>} and
\tcode{F} be \tcode{decltype(f)}.
Let \tcode{T} be \tcode{remove_reference_t<U>}.

\pnum
\constraints
Expand All @@ -15360,7 +15358,7 @@
\pnum
\mandates
If \tcode{is_pointer_v<F> || is_member_pointer_v<F>} is \tcode{true},
then \tcode{f != nullptr} is \tcode{true}.
then \tcode{f.value != nullptr} is \tcode{true}.

\pnum
\effects
Expand All @@ -15370,27 +15368,24 @@
such that
\tcode{\placeholder{thunk}(\exposid{bound-entity}, \placeholder{call-args}...)}
is expression-equivalent\iref{defns.expression.equivalent} to
\tcode{invoke_r<R>(f, static_cast<\cv{} T\&>(obj), \placeholder{call-args}...)}.
\tcode{invoke_r<R>(f.value, static_cast<\cv{} T\&>(obj), \placeholder{call-args}...)}.
\end{itemdescr}

\indexlibraryctor{function_ref}%
\begin{itemdecl}
template<auto f, class T>
constexpr function_ref(constant_arg_t<f>, @\cv{}@ T* obj) noexcept;
template<auto c, class F>
constexpr function_ref(constant_wrapper<c, F> f, @\cv{}@ T* obj) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
Let \tcode{F} be \tcode{decltype(f)}.

\pnum
\constraints
\tcode{\exposid{is-invocable-using}<const F\&, \cv{} T*>} is \tcode{true}.

\pnum
\mandates
If \tcode{is_pointer_v<F> || is_member_pointer_v<F>} is \tcode{true},
then \tcode{f != nullptr} is \tcode{true}.
then \tcode{f.value != nullptr} is \tcode{true}.

\pnum
\expects
Expand All @@ -15405,7 +15400,7 @@
such that
\tcode{\placeholder{thunk}(\exposid{bound-entity}, \placeholder{call-args}...)}
is expression-equivalent\iref{defns.expression.equivalent} to
\tcode{invoke_r<R>(f, obj, \placeholder{call-args}...)}.
\tcode{invoke_r<R>(f.value, obj, \placeholder{call-args}...)}.
\end{itemdescr}

\indexlibrarymember{operator=}{function_ref}%
Expand All @@ -15419,7 +15414,7 @@
\begin{itemize}
\item \tcode{T} is not the same type as \tcode{function_ref},
\item \tcode{is_pointer_v<T>} is \tcode{false}, and
\item \tcode{T} is not a specialization of \tcode{constant_arg_t}.
\item \tcode{T} is not a specialization of \tcode{constant_wrapper}.
\end{itemize}
\end{itemdescr}

Expand Down Expand Up @@ -15451,13 +15446,13 @@
\end{itemdescr}

\begin{itemdecl}
template<auto f>
function_ref(constant_arg_t<f>) -> function_ref<@\seebelow@>;
template<auto c, class F0>
function_ref(constant_wrapper<c, F0>) -> function_ref<@\seebelow@>;
\end{itemdecl}

\begin{itemdescr}
\pnum
Let \tcode{F} be \tcode{remove_pointer_t<decltype(f)>}.
Let \tcode{F} be \tcode{remove_pointer_t<F0>}.

\pnum
\constraints
Expand All @@ -15469,14 +15464,11 @@
\end{itemdescr}

\begin{itemdecl}
template<auto f, class T>
function_ref(constant_arg_t<f>, T&&) -> function_ref<@\seebelow@>;
template<auto c, class F, class T>
function_ref(constant_wrapper<c, F>, T&&) -> function_ref<@\seebelow@>;
\end{itemdecl}

\begin{itemdescr}
\pnum
Let \tcode{F} be \tcode{decltype(f)}.

\pnum
\constraints
%FIXME: R and E should be defined outside of these constraints.
Expand Down
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