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cor3ntin committed Oct 9, 2024
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10 changes: 10 additions & 0 deletions src/D2019.tex
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Expand Up @@ -39,6 +39,16 @@ \subsection{R7}
\begin{itemize}
\item Improve wording following St Louis guidance.
\item Remove \tcode{char8_t} support - pending SG16 discussion.

\tcode{char8_t} support was present based on the assumption that, given that
a thread name is textual data, and because it matches the capability of underlying platforms,
we should support Unicode. And there is no point in supporting \tcode{wchar_t/char16_t/char32_t} as
we do a copy in all cases and we should move away from having every standard interface accepting text
being quituplicated.
However this was either not presented, or at least not presented enough.
We therefore removed the support in the interest of incremental progress and will bring it back in the separate paper.
(We are keeping an interface taking \tcode{basic_string_view<T>} rather than \tcode{string_view} in order to facilitate such future evolution in a non abi-breaking manner.)

\end{itemize}
\subsection{R6}

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201 changes: 103 additions & 98 deletions src/D2841.tex
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Expand Up @@ -5,8 +5,8 @@
\documentclass{wg21}

\title{Concept and variable-template template-parameters}
\docnumber{D2841R4}
\audience{EWG}
\docnumber{D2841R5}
\audience{CWG}
\author{Corentin Jabot}{[email protected]}
\authortwo{Gašper Ažman}{[email protected]}
\authorthree{James Touton}{[email protected]}
Expand Down Expand Up @@ -48,6 +48,12 @@ \section{Abstract}

\section{Revisions}

\subsection{R5}

\begin{itemize}
\item Wording fixes.
\end{itemize}

\subsection{R4}

\begin{itemize}
Expand All @@ -61,7 +67,8 @@ \subsection{R4}
\item Use the prose terminology and typography in more places
\item Improve the sections introducing template parameters and pack thereof
\item Change the grammar of template arguments not to use id-expression
\item Address typos and other CWG feedback.
\item Introduce "concept dependent constraints"
\item Address CWG feedback.
\end{itemize}

\subsection{R3}
Expand Down Expand Up @@ -2083,15 +2090,15 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem

\rSec3[temp.constr.concept]{Concept-dependent constraint}

A \defn{concept-dependent constraint} is formed from a concept-id \tcode{C<A1, ..., A>}
A \defn{concept-dependent constraint} is formed from a concept-id \tcode{C<A1, ..., An>}
where \tcode{C} names a concept template parameter.

Two concept-dependent constraints are equivalent if their concept-id are equivalent.
Two concept-dependent constraints are equivalent if their concept-ids are equivalent.


To determine if a concept-dependent constraint is satisfied, template arguments are first substituted into its
\tcode{concept-id} \tcode{C}. If substitution results in an invalid type or expression in the immediate context of the constraint ([temp.deduct.general]), the constraint is not satisfied.
Otherwise, let \tcode{E} be the normal form of \tcode{C}.
concept-id \tcode{C<A1, ..., An>}. If substitution results in an invalid concept-id in the immediate context of the constraint ([temp.deduct.general]), the constraint is not satisfied.
Otherwise, let \tcode{E} be the normal form of \tcode{C<A1, ..., An>} after substitution.
The constraint is satisfied if \tcode{E} is satisfied.


Expand Down Expand Up @@ -2131,11 +2138,11 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
\begin{addedblock}
\item When \tcode{C} names a concept template parameter,
the normal form of a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>} is
a concept-dependent constraint whose \grammarterm{concept-id} is \tcode{C<A$_1$, A$_2$, ..., A$_n$>}.
a concept-dependent constraint whose concept-id is \tcode{C<A$_1$, A$_2$, ..., A$_n$>}.
\end{addedblock}

\item
\added{Otherwise}, The normal form of a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>}
\added{Otherwise}, \changed{The}{the} normal form of a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>}
is the normal form of the \grammarterm{constraint-expression} of \tcode{C},
after substituting \tcode{A$_1$, A$_2$, ..., A$_n$} for
\tcode{C}{'s} respective template parameters in the
Expand Down Expand Up @@ -2244,18 +2251,18 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
template <typename T>
concept C = true;

template <template <typename T, template<typename...> concept> concept CT>
concept CC= CT<T>;
template <typename T, template<typename, template <typename> concept> concept CT>
concept CC = CT<T>;

template <template <typename U, template<typename...> concept> concept CT>
template <typename U, template<typename, template <typename> concept> concept CT>
void f() requires CT<U*, C>;

template <typename U>
void g() requires CC<U*, C>;
\end{codeblock}

The normal form of the associated constraints of \tcode{f} is the
atomic constraint \tcode{CT<T, C>}.
concept-dependent constraint \tcode{CT<T, C>}.
%%(with mapping $\tcode{T} \mapsto \tcode{U*}, ????$)

The normal form of the associated constraints of \tcode{g} is the
Expand Down Expand Up @@ -2361,7 +2368,7 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
%the atomic constraint
%whose expression is \tcode{E} and
%whose parameter mapping is the identity mapping.
%\end{itemize}
%\end{end}
%
%\pnum
%The process of obtaining the normal form of a
Expand All @@ -2382,8 +2389,6 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
\rSec2[temp.constr.order]{Partial ordering by constraints}
\indextext{subsume|see{constraint, subsumption}}

\rSec2[temp.constr.order]{Partial ordering by constraints}

\pnum
\indextext{subsume|see{constraint, subsumption}}
A constraint $P$ \defnx{subsumes}{constraint!subsumption} a constraint $Q$
Expand All @@ -2409,8 +2414,8 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem

\begin{addedblock}
\item a concept-dependent constraint $A$ subsumes
another concept-dependent expanded constraint $B$
if their concept-id are equivalent.
another concept-dependent constraint $B$
if their concept-ids are equivalent.
\end{addedblock}

\end{itemize}
Expand Down Expand Up @@ -2441,7 +2446,7 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
\begin{addedblock}

The associated constraints \tcode{C} of a declaration \tcode{D} are \emph{eligible for subsumption}
unless \tcode{C} contains an atomic constraint that names a \grammarterm{concept-tt-parameter}.
unless \tcode{C} contains a concept-dependent constraint.

\end{addedblock}

Expand Down Expand Up @@ -2602,85 +2607,85 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem

\textcolor{noteclr}{[...]}

\rSec2[temp.inst]{Implicit instantiation}

\pnum
\removed{The \grammarterm{type-constraint}{s} and \grammarterm{requires-clause}
of a template specialization or member function
are not instantiated along with the specialization or function itself,
even for a member function of a local class;}
\begin{addedblock}
When instantiating a template specialization or member function, each concept-dependent constraint
in its \grammarterm{type-constraint}{s} and \grammarterm{requires-clause}
is replaced by its substited normal form [temp.constr.concept].
No further instantiation of the constraints are performed, even for a member function of a local class;
\end{addedblock}
substitution into the atomic constraints formed from them is instead performed
as specified in [temp.constr.decl] and [temp.constr.atomic]
when determining whether the constraints are satisfied
or as specified in [temp.constr.dec] when comparing declarations.
\begin{note}
The satisfaction of constraints is determined during
template argument deduction \iref{temp.deduct} and
overload resolution \iref{over.match}.
\end{note}
\begin{example}
\begin{codeblock}
template<typename T> concept C = sizeof(T) > 2;
template<typename T> concept D = C<T> && sizeof(T) > 4;

template<typename T> struct S {
S() requires C<T> { } // \#1
S() requires D<T> { } // \#2
};

S<char> s1; // error: no matching constructor
S<char[8]> s2; // OK, calls \#2
\end{codeblock}
When \tcode{S<char>} is instantiated, both constructors are part of the
specialization. Their constraints are not satisfied, and
they suppress the implicit declaration of a default constructor for
\tcode{S<char>} \iref{class.default.ctor}, so there is no viable constructor
for \tcode{s1}.
\end{example}
\begin{example}
\begin{codeblock}
template<typename T> struct S1 {
template<typename U>
requires false
struct Inner1; // ill-formed, no diagnostic required
};

template<typename T> struct S2 {
template<typename U>
requires (sizeof(T[-(int)sizeof(T)]) > 1)
struct Inner2; // ill-formed, no diagnostic required
};
\end{codeblock}
The class \tcode{S1<T>::Inner1} is ill-formed, no diagnostic required, because
it has no valid specializations.
\tcode{S2} is ill-formed, no diagnostic required, since no substitution into
the constraints of its \tcode{Inner2} template would result in a valid
expression.
\end{example}

\begin{addedblock}
\begin{example}
\begin{codeblock}
template<typename T> concept C = true;
template<typename T> concept D = true && true;
template <template <typename> concept TT>
struct S {
template <typename T>
void f(T) requires C<T>; // \#1
template <typename T>
void f(T) requires TT<T> // \#2
};
S<D>{}.f(42); // calls \#2: D<int> subsumes C<int>
\end{codeblock}
\end{example}
\end{addedblock}
\textcolor{noteclr}{[...]}
%\rSec2[temp.inst]{Implicit instantiation}
%
%\pnum
%\removed{The \grammarterm{type-constraint}{s} and \grammarterm{requires-clause}
%of a template specialization or member function
%are not instantiated along with the specialization or function itself,
%even for a member function of a local class;}
%\begin{addedblock}
%When instantiating a template specialization or member function, each concept-dependent constraint
%in its \grammarterm{type-constraint}{s} and \grammarterm{requires-clause}
%is replaced by its substited normal form [temp.constr.concept].
%No further instantiation of the constraints are performed, even for a member function of a local class;
%\end{addedblock}
%substitution into the atomic constraints formed from them is instead performed
%as specified in [temp.constr.decl] and [temp.constr.atomic]
%when determining whether the constraints are satisfied
%or as specified in [temp.constr.dec] when comparing declarations.
%\begin{note}
% The satisfaction of constraints is determined during
% template argument deduction \iref{temp.deduct} and
% overload resolution \iref{over.match}.
%\end{note}
%\begin{example}
%\begin{codeblock}
% template<typename T> concept C = sizeof(T) > 2;
% template<typename T> concept D = C<T> && sizeof(T) > 4;
%
% template<typename T> struct S {
% S() requires C<T> { } // \#1
% S() requires D<T> { } // \#2
% };
%
% S<char> s1; // error: no matching constructor
% S<char[8]> s2; // OK, calls \#2
%\end{codeblock}
%When \tcode{S<char>} is instantiated, both constructors are part of the
%specialization. Their constraints are not satisfied, and
%they suppress the implicit declaration of a default constructor for
%\tcode{S<char>} \iref{class.default.ctor}, so there is no viable constructor
%for \tcode{s1}.
%\end{example}
%\begin{example}
%\begin{codeblock}
% template<typename T> struct S1 {
% template<typename U>
% requires false
% struct Inner1; // ill-formed, no diagnostic required
% };
%
% template<typename T> struct S2 {
% template<typename U>
% requires (sizeof(T[-(int)sizeof(T)]) > 1)
% struct Inner2; // ill-formed, no diagnostic required
% };
%\end{codeblock}
%The class \tcode{S1<T>::Inner1} is ill-formed, no diagnostic required, because
%it has no valid specializations.
%\tcode{S2} is ill-formed, no diagnostic required, since no substitution into
%the constraints of its \tcode{Inner2} template would result in a valid
%expression.
%\end{example}
%
%\begin{addedblock}
%\begin{example}
%\begin{codeblock}
%template<typename T> concept C = true;
%template<typename T> concept D = true && true;
%template <template <typename> concept TT>
%struct S {
% template <typename T>
% void f(T) requires C<T>; // \#1
% template <typename T>
% void f(T) requires TT<T> // \#2
%};
%S<D>{}.f(42); // calls \#2: D<int> subsumes C<int>
%\end{codeblock}
%\end{example}
%\end{addedblock}
%\textcolor{noteclr}{[...]}

\rSec3[temp.over.link]{Function template overloading}
\textcolor{noteclr}{[...]}
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