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[2026-03 LWG Motion 18] P3961R1 Less double indirection in function_ref (RU-220) #8901

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[2026-03 LWG Motion 18] P3961R1 Less double indirection in function_ref (RU-220) #8901

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2 changes: 1 addition & 1 deletion source/support.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -711,7 +711,7 @@
#define @\defnlibxname{cpp_lib_freestanding_utility}@ 202306L // freestanding, also in \libheader{utility}
#define @\defnlibxname{cpp_lib_freestanding_variant}@ 202311L // freestanding, also in \libheader{variant}
#define @\defnlibxname{cpp_lib_fstream_native_handle}@ 202306L // also in \libheader{fstream}
#define @\defnlibxname{cpp_lib_function_ref}@ 202511L // freestanding, also in \libheader{functional}
#define @\defnlibxname{cpp_lib_function_ref}@ 202603L // freestanding, also in \libheader{functional}
#define @\defnlibxname{cpp_lib_gcd_lcm}@ 201606L // freestanding, also in \libheader{numeric}
#define @\defnlibxname{cpp_lib_generator}@ 202207L // also in \libheader{generator}
#define @\defnlibxname{cpp_lib_generic_associative_lookup}@ 201304L // also in \libheader{map}, \libheader{set}
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57 changes: 51 additions & 6 deletions source/utilities.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -13835,10 +13835,12 @@

\pnum
Let \tcode{t} be an object of a type that is a specialization of
\tcode{function}, \tcode{copyable_function}, or \tcode{move_only_function},
\tcode{function}, \tcode{copyable_function}, \tcode{move_only_function},
or \tcode{function_ref},
such that the target object \tcode{x} of \tcode{t} has a type that
is a specialization of
\tcode{function}, \tcode{copyable_function}, or \tcode{move_only_function}.
\tcode{function}, \tcode{copyable_function}, \tcode{move_only_function},
or \tcode{function_ref}.
Each argument of the
invocation of \tcode{x} evaluated as part of the invocation of \tcode{t}
may alias an argument in the same position in the invocation of \tcode{t} that
Expand Down Expand Up @@ -15253,6 +15255,25 @@
\end{codeblock}
\end{itemdescr}

\indextext{function_ref::is-convertible-from-specialization@\tcode{function_ref::\exposid{is-convertible-from-\brk{}specialization}}}%
\begin{itemdecl}
template<class F>
static constexpr bool @\exposid{is-convertible-from-specialization}@ = @\seebelow@;
\end{itemdecl}

\begin{itemdescr}
\pnum
If \tcode{F} denotes a specialization
\tcode{function_ref<R(Args...) \cvqual{cv2} noexcept(\placeholder{noex2})>}
for some placeholders \cvqual{cv2} and \placeholder{noex2},
\tcode{\exposid{is-convertible-from-specialization}<F>} is equal to:
\begin{codeblock}
is_convertible_v<R(&)(Args...) noexcept(@\placeholder{noex2}@), R(&)(Args...) noexcept(@\placeholder{noex}@)> &&
is_convertible_v<int @\cv{}@&, int @\cvqual{cv2}@&>.
\end{codeblock}
Otherwise, \tcode{\exposid{is-convertible-from-specialization}<F>} is \tcode{false}.
\end{itemdescr}

\indexlibraryctor{function_ref}%
\begin{itemdecl}
template<class F> function_ref(F* f) noexcept;
Expand Down Expand Up @@ -15300,13 +15321,37 @@

\pnum
\effects
Initializes
\exposid{bound-entity} with \tcode{addressof(f)}, and
If \tcode{\exposid{is-convertible-from-specialization}<remove_cv_t<T>>}
is \tcode{false},
initializes
\exposid{bound-en\-tity} with \tcode{addressof(f)}, and
\exposid{thunk-ptr} with the address of a function \tcode{\placeholder{thunk}}
such that
\tcode{\placeholder{thunk}(\exposid{bound-entity}, \placeholder{call-args}...)}
\tcode{\placeholder{thunk}(\exposid{bound-\brk{}entity}, \placeholder{call-args}...)}
is expression-equivalent\iref{defns.expression.equivalent} to
\tcode{invoke_r<R>(static_cast<\cv{} T\&>(f), \placeholder{call-args}...)}.
Otherwise, initializes \exposid{bound-entity}
with the value of \tcode{f.\exposid{bound-entity}} and
\exposid{thunk-\brk{}ptr} with the value of \tcode{f.\exposid{thunk-ptr}}.

\pnum
\remarks
If \tcode{remove_cvref_t<F>} is a specialization of \tcode{function_ref},
an implementation may initialize \exposid{bound-entity}
with the value of \tcode{f.\exposid{bound-entity}} and \exposid{thunk-ptr}
with the value of \tcode{f.\exposid{thunk-ptr}}.
\begin{example}
\begin{codeblock}
void f1(std::string);
void f2(std::string);

function_ref<void(std::string)> r1(&f1);
function_ref<void(std::string&&)> r2(r1);
r2(""); // \tcode{f1} is invoked
r1 = &f2;
r2(""); // it is unspecified if \tcode{f1} or \tcode{f2} is invoked
\end{codeblock}
\end{example}
\end{itemdescr}

\indexlibraryctor{function_ref}%
Expand Down Expand Up @@ -15417,7 +15462,7 @@
\pnum
\constraints
\begin{itemize}
\item \tcode{T} is not the same type as \tcode{function_ref},
\item \tcode{\exposid{is-convertible-from-specialization}<T>} is \tcode{false},
\item \tcode{is_pointer_v<T>} is \tcode{false}, and
\item \tcode{T} is not a specialization of \tcode{constant_arg_t}.
\end{itemize}
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