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P2404R3 Move-only types for equality_comparable_with, totally_ordered_with, and three_way_comparable_with
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‎source/compatibility.tex

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@@ -133,6 +133,33 @@
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Valid \CppXX{} code that \tcode{\#include}{s} headers with these names may be
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invalid in this revision of \Cpp{}.
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\rSec2[diff.cpp20.concepts]{\ref{concepts}: concepts library}
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\diffref{cmp.concept,concept.equalitycomparable,concept.totallyordered}
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\change
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Replace \tcode{common_reference_with} in \tcode{three_way_comparable_with},
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\tcode{equality_comparable_with}, and \tcode{totally_ordered_with}
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with an exposition-only concept.
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\rationale
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Allow uncopyable, but movable, types to model these concepts.
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\effect
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Valid \CppXX{} code relying on subsumption
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with \tcode{common_reference_with}
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may fail to compile in this revision of \Cpp{}. For example:
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\begin{codeblock}
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template<class T, class U>
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requires @\libconcept{equality_comparable_with}@<T, U>
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bool attempted_equals(const T&, const U& u); // previously selected overload
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template<class T, class U>
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requires @\libconcept{common_reference_with}@<const remove_reference_t<T>&, const remove_reference_t<U>&>
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bool attempted_equals(const T& t, const U& u); // ambiguous overload; previously
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// rejected by partial ordering
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bool test(shared_ptr<int> p) {
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return attempted_equals(p, nullptr); // ill-formed; previously well-formed
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}
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\end{codeblock}
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\rSec2[diff.cpp20.utilities]{\ref{utilities}: general utilities library}
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\diffref{format}

‎source/concepts.tex

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@@ -849,6 +849,16 @@
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Subclause \ref{concepts.compare} describes concepts that establish relationships and orderings
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on values of possibly differing object types.
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\pnum
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Given an expression \tcode{E} and a type \tcode{C},
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let \tcode{\exposid{CONVERT_TO_LVALUE}<C>(E)} be:
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\begin{itemize}
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\item
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\tcode{static_cast<const C\&>(as_const(E))} if that is a valid expression, and
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\item
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\tcode{static_cast<const C\&>(std::move(E))} otherwise.
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\end{itemize}
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\rSec2[concept.booleantestable]{Boolean testability}
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\pnum
@@ -985,6 +995,42 @@
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model \exposconcept{boolean-testable}.
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\end{example}
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\rSec2[concept.comparisoncommontype]{Comparison common types}
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\begin{itemdecl}
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template<class T, class U, class C = common_reference_t<const T&, const U&>>
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concept @\defexposconcept{comparison-common-type-with-impl}@ = // \expos
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@\libconcept{same_as}@<common_reference_t<const T&, const U&>,
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common_reference_t<const U&, const T&>> &&
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requires {
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requires @\libconcept{convertible_to}@<const T&, const C&> || @\libconcept{convertible_to}@<T, const C&>;
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requires @\libconcept{convertible_to}@<const U&, const C&> || @\libconcept{convertible_to}@<U, const C&>;
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};
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template<class T, class U>
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concept @\defexposconcept{comparison-common-type-with}@ = // \expos
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@\exposconcept{comparison-common-type-with-impl}@<remove_cvref_t<T>, remove_cvref_t<U>>;
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\end{itemdecl}
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\pnum
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Let \tcode{C} be \tcode{common_reference_t<const T\&, const U\&>}.
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Let \tcode{t1} and \tcode{t2} be equality-preserving expressions
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that are lvalues of type \tcode{remove_cvref_t<T>}, and
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let \tcode{u1} and \tcode{u2} be equality-preserving expressions
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that are lvalues of type \tcode{remove_cvref_t<U>}.
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\tcode{T} and \tcode{U} model
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\tcode{\exposconcept{comparison-common-type-with}<T, U>} only if:
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\begin{itemize}
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\item
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\tcode{\exposid{CONVERT_TO_LVALUE}<C>(t1)} equals
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\tcode{\exposid{CONVERT_TO_LVALUE}<C>(t2)}
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if and only if \tcode{t1} equals \tcode{t2}, and
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\item
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\tcode{\exposid{CONVERT_TO_LVALUE}<C>(u1)} equals
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\tcode{\exposid{CONVERT_TO_LVALUE}<C>(u2)}
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if and only if \tcode{u1} equals \tcode{u2}
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\end{itemize}
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9881034
\rSec2[concept.equalitycomparable]{Concept \cname{equality_comparable}}
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\begin{itemdecl}
@@ -1039,7 +1085,7 @@
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template<class T, class U>
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concept @\deflibconcept{equality_comparable_with}@ =
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@\libconcept{equality_comparable}@<T> && @\libconcept{equality_comparable}@<U> &&
1042-
@\libconcept{common_reference_with}@<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
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@\exposconcept{comparison-common-type-with}@<T, U> &&
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@\libconcept{equality_comparable}@<
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common_reference_t<
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const remove_reference_t<T>&,
@@ -1050,15 +1096,21 @@
10501096
\begin{itemdescr}
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\pnum
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Given types \tcode{T} and \tcode{U},
1053-
let \tcode{t} be an lvalue of type \tcode{const remove_reference_t<T>},
1054-
\tcode{u} be an lvalue of type \tcode{const remove_reference_t<U>},
1055-
and \tcode{C} be:
1099+
let \tcode{t} and \tcode{t2} be lvalues
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denoting distinct equal objects of types \tcode{const remove_reference_t<T>} and
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\tcode{remove_cvref_t<T>}, respectively,
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let \tcode{u} and \tcode{u2} be lvalues
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denoting distinct equal objects of types \tcode{const remove_reference_t<U>} and
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\tcode{remove_cvref_t<U>}, respectively, and
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let \tcode{C} be:
10561106
\begin{codeblock}
10571107
common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>
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\end{codeblock}
10591109
\tcode{T} and \tcode{U} model
10601110
\tcode{\libconcept{equality_comparable_with}<T, U>} only if
1061-
\tcode{bool(t == u) == bool(C(t) == C(u))}.
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\begin{codeblock}
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bool(t == u) == bool(@\exposid{CONVERT_TO_LVALUE}@<C>(t2) == @\exposid{CONVERT_TO_LVALUE}@<C>(u2))
1113+
\end{codeblock}
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\end{itemdescr}
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\rSec2[concept.totallyordered]{Concept \cname{totally_ordered}}
@@ -1101,24 +1153,27 @@
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\begin{itemdescr}
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\pnum
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Given types \tcode{T} and \tcode{U},
1104-
let \tcode{t} be an lvalue of type \tcode{const remove_reference_t<T>},
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\tcode{u} be an lvalue of type \tcode{const remove_reference_t<U>},
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and \tcode{C} be:
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let \tcode{t} and \tcode{t2} be lvalues
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denoting distinct equal objects of types \tcode{const remove_reference_t<T>} and
1158+
\tcode{remove_cvref_t<T>}, respectively,
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let \tcode{u} and \tcode{u2} be lvalues
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denoting distinct equal objects of types \tcode{const remove_reference_t<U>} and
1161+
\tcode{remove_cvref_t<U>}, respectively, and
1162+
let \tcode{C} be:
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\begin{codeblock}
11081164
common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>
11091165
\end{codeblock}
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\tcode{T} and \tcode{U} model
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\tcode{\libconcept{totally_ordered_with}<T, U>} only if
1112-
11131168
\begin{itemize}
1114-
\item \tcode{bool(t < u) == bool(C(t) < C(u)).}
1115-
\item \tcode{bool(t > u) == bool(C(t) > C(u)).}
1116-
\item \tcode{bool(t <= u) == bool(C(t) <= C(u)).}
1117-
\item \tcode{bool(t >= u) == bool(C(t) >= C(u)).}
1118-
\item \tcode{bool(u < t) == bool(C(u) < C(t)).}
1119-
\item \tcode{bool(u > t) == bool(C(u) > C(t)).}
1120-
\item \tcode{bool(u <= t) == bool(C(u) <= C(t)).}
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\item \tcode{bool(u >= t) == bool(C(u) >= C(t)).}
1169+
\item \tcode{bool(t < u) == bool(\exposid{CONVERT_TO_LVALUE}<C>(t2) < \exposid{CONVERT_TO_LVALUE}<C>(u2))}.
1170+
\item \tcode{bool(t > u) == bool(\exposid{CONVERT_TO_LVALUE}<C>(t2) > \exposid{CONVERT_TO_LVALUE}<C>(u2))}.
1171+
\item \tcode{bool(t <= u) == bool(\exposid{CONVERT_TO_LVALUE}<C>(t2) <= \exposid{CONVERT_TO_LVALUE}<C>(u2))}.
1172+
\item \tcode{bool(t >= u) == bool(\exposid{CONVERT_TO_LVALUE}<C>(t2) >= \exposid{CONVERT_TO_LVALUE}<C>(u2))}.
1173+
\item \tcode{bool(u < t) == bool(\exposid{CONVERT_TO_LVALUE}<C>(u2) < \exposid{CONVERT_TO_LVALUE}<C>(t2))}.
1174+
\item \tcode{bool(u > t) == bool(\exposid{CONVERT_TO_LVALUE}<C>(u2) > \exposid{CONVERT_TO_LVALUE}<C>(t2))}.
1175+
\item \tcode{bool(u <= t) == bool(\exposid{CONVERT_TO_LVALUE}<C>(u2) <= \exposid{CONVERT_TO_LVALUE}<C>(t2))}.
1176+
\item \tcode{bool(u >= t) == bool(\exposid{CONVERT_TO_LVALUE}<C>(u2) >= \exposid{CONVERT_TO_LVALUE}<C>(t2))}.
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\end{itemize}
11231178
\end{itemdescr}
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‎source/support.tex

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@@ -589,7 +589,7 @@
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#define @\defnlibxname{cpp_lib_chrono_udls}@ 201304L // also in \libheader{chrono}
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#define @\defnlibxname{cpp_lib_clamp}@ 201603L // also in \libheader{algorithm}
591591
#define @\defnlibxname{cpp_lib_complex_udls}@ 201309L // also in \libheader{complex}
592-
#define @\defnlibxname{cpp_lib_concepts}@ 202002L // also in \libheader{concepts}
592+
#define @\defnlibxname{cpp_lib_concepts}@ 202207L // also in \libheader{concepts}, \libheader{compare}
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#define @\defnlibxname{cpp_lib_constexpr_algorithms}@ 201806L // also in \libheader{algorithm}
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#define @\defnlibxname{cpp_lib_constexpr_charconv}@ 202207L // also in \libheader{charconv}
595595
#define @\defnlibxname{cpp_lib_constexpr_cmath}@ 202202L // also in \libheader{cmath}, \libheader{cstdlib}
@@ -4830,7 +4830,7 @@
48304830
concept @\deflibconcept{three_way_comparable_with}@ =
48314831
@\libconcept{three_way_comparable}@<T, Cat> &&
48324832
@\libconcept{three_way_comparable}@<U, Cat> &&
4833-
@\libconcept{common_reference_with}@<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
4833+
@\exposconcept{comparison-common-type-with}@<T, U> &&
48344834
@\libconcept{three_way_comparable}@<
48354835
common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
48364836
@\exposconcept{weakly-equality-comparable-with}@<T, U> &&
@@ -4842,11 +4842,17 @@
48424842
\end{codeblock}
48434843

48444844
\pnum
4845-
Let \tcode{t} and \tcode{u} be lvalues
4845+
Let \tcode{t} and \tcode{t2} be lvalues
4846+
denoting distinct equal objects
48464847
of types \tcode{const remove_reference_t<T>} and
4847-
\tcode{const remove_reference_t<U>}, respectively.
4848+
\tcode{remove_cvref_t<T>}, respectively, and
4849+
let \tcode{u} and \tcode{u2} be lvalues denoting distinct equal objects
4850+
of types \tcode{const remove_reference_t<U>} and
4851+
\tcode{remove_cvref_t<U>}, respectively.
48484852
Let \tcode{C} be
48494853
\tcode{common_reference_t<const remove_reference_t<T>\&, const remove_reference_t<U>\&>}.
4854+
Let \tcode{\exposid{CONVERT_TO_LVALUE}<C>(E)} be defined
4855+
as in \ref{concepts.compare.general}.
48504856
\tcode{T}, \tcode{U}, and \tcode{Cat}
48514857
model \tcode{\libconcept{three_way_comparable_with}<T, U, Cat>} only if:
48524858
\begin{itemize}
@@ -4859,7 +4865,8 @@
48594865
\item
48604866
\tcode{(t <=> u != 0) == bool(t != u)} is \tcode{true},
48614867
\item
4862-
\tcode{Cat(t <=> u) == Cat(C(t) <=> C(u))} is \tcode{true},
4868+
\tcode{Cat(t <=> u) == Cat(\exposid{CONVERT_TO_LVALUE}<C>(t2) <=>
4869+
\exposid{CONVERT_TO_LVALUE}<C>(u2))} is \tcode{true},
48634870
\item
48644871
\tcode{(t <=> u < 0) == bool(t < u)} is \tcode{true},
48654872
\item

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