P2167R3 Improved Proposed Wording for LWG 2114 (contextually convertible to bool)

Editorial notes:
* Minor consistency fixes around the use of "each".
* Some requested wording changes have already been applied
  as part of LWG 2114.

Fixes NB US 073 (C++23 CD).

Author: jensmaurer

source/algorithms.tex

@@ -151,13 +151,13 @@
 whenever an algorithm expects a function object\iref{function.objects} that,
 when applied to the result of dereferencing the corresponding iterator,
 returns a value testable as \tcode{true}.
-In other words,
-if an algorithm takes \tcode{Predicate pred} as its argument and
+If an algorithm takes \tcode{Predicate pred} as its argument and
 \tcode{first} as its iterator argument with value type \tcode{T},
-it should work correctly in the construct
-\tcode{pred(*first)} contextually converted to \tcode{bool}\iref{conv}.
+the expression \tcode{pred(*first)} shall be well-formed and
+the type \tcode{decltype(pred(*first))} shall model
+\exposconcept{boolean-testable}\iref{concept.booleantestable}.
 The function object \tcode{pred} shall not apply any non-constant function
-through the dereferenced iterator.
+through its argument.
 Given a glvalue \tcode{u} of type (possibly \keyword{const}) \tcode{T}
 that designates the same object as \tcode{*first},
 \tcode{pred(u)} shall be a valid expression
@@ -170,19 +170,21 @@
 to dereferencing an iterator and type \tcode{T}
 when \tcode{T} is part of the signature,
 returns a value testable as \tcode{true}.
-In other words,
-if an algorithm takes \tcode{BinaryPredicate binary_pred} as its argument and
+If an algorithm takes \tcode{BinaryPredicate binary_pred} as its argument and
 \tcode{first1} and \tcode{first2} as its iterator arguments
 with respective value types \tcode{T1} and \tcode{T2},
-it should work correctly in the construct
-\tcode{binary_pred(*first1, *first2)} contextually converted to \tcode{bool}\iref{conv}.
+the expression \tcode{binary_pred(*first1, *first2)} shall be well-formed and
+the type \tcode{decltype(binary_pred(*first1, *first2))} shall model
+\exposconcept{boolean-testable}.
 Unless otherwise specified,
 \tcode{BinaryPredicate} always takes the first iterator's \tcode{value_type}
 as its first argument, that is, in those cases when \tcode{T value}
-is part of the signature, it should work correctly in the construct
-\tcode{binary_pred(*first1, value)} contextually converted to \tcode{bool}\iref{conv}.
+is part of the signature,
+the expression \tcode{binary_pred(*first1, value)} shall be well-formed and
+the type \tcode{decltype(binary_pred(*first1, value))} shall model
+\exposconcept{boolean-testable}.
 \tcode{binary_pred} shall not apply any non-constant function
-through the dereferenced iterators.
+through any of its arguments.
 Given a glvalue \tcode{u} of type (possibly \keyword{const}) \tcode{T1}
 that designates the same object as \tcode{*first1}, and
 a glvalue \tcode{v} of type (possibly \keyword{const}) \tcode{T2}
@@ -6569,7 +6571,7 @@
 named \tcode{BinaryPredicate}~\iref{algorithms.requirements}.
 The return value of the function call operation
 applied to an object of type \tcode{Compare},
-when contextually converted to \tcode{bool}\iref{conv},
+when converted to \tcode{bool},
 yields \tcode{true}
 if the first argument of the call is less than the second, and
 \tcode{false} otherwise.

source/containers.tex

@@ -425,7 +425,7 @@
 
 \pnum
 \result
-Convertible to \tcode{bool}.
+\tcode{bool}.
 
 \pnum
 \returns
@@ -536,7 +536,7 @@
 \begin{itemdescr}
 \pnum
 \result
-Convertible to \tcode{bool}.
+\tcode{bool}.
 
 \pnum
 \returns

source/iterators.tex

@@ -2095,7 +2095,7 @@
                     &                       &   \chdr{semantics}    &   \rhdr{pre-/post-condition}   \\ \capsep
 \endhead
 \tcode{a != b}                  &
- contextually convertible to \tcode{bool}    &
+ \tcode{decltype(a != b)} models \exposconceptx{boolean-test\-able}{boolean-testable}  &
  \tcode{!(a == b)}                              &
  \expects \orange{a}{b} is in the domain of \tcode{==}. \\ \rowsep
 
@@ -2383,25 +2383,21 @@
  \tcode{*(a + n)}   &   \\ \rowsep
 
 \tcode{a < b}       &
- contextually
- convertible to \tcode{bool}    &
+ \tcode{decltype(a < b)} models \exposconceptx{boolean-test\-able}{boolean-testable}    &
  \effects Equivalent to: \tcode{return b - a > 0;}  &
  \tcode{<} is a total ordering relation \\ \rowsep
 
 \tcode{a > b}       &
- contextually
- convertible to \tcode{bool}    &
+ \tcode{decltype(a > b)} models \exposconceptx{boolean-test\-able}{boolean-testable}    &
  \tcode{b < a}      &
  \tcode{>} is a total ordering relation opposite to \tcode{<}.  \\ \rowsep
 
 \tcode{a >= b}      &
- contextually
- convertible to \tcode{bool}    &
+ \tcode{decltype(a >= b)} models \exposconceptx{boolean-test\-able}{boolean-testable}    &
  \tcode{!(a < b)}   &   \\ \rowsep
 
 \tcode{a <= b}      &
- contextually
- convertible to \tcode{bool}.    &
+ \tcode{decltype(a <= b)} models \exposconceptx{boolean-test\-able}{boolean-testable}    &
  \tcode{!(a > b)}   &   \\
 \end{libreqtab4b}
 

source/lib-intro.tex

@@ -1622,7 +1622,7 @@
 \topline
 \hdstyle{Expression}  &   \hdstyle{Return type} &   \rhdr{Requirement} \\ \capsep
 \tcode{a == b}  &
-convertible to \tcode{bool} &
+\tcode{decltype(a == b)} models \exposconceptx{boolean-testa\-ble}{boolean-testable} &
 \tcode{==} is an equivalence relation,
 that is, it has the following properties:
 \begin{itemize}
@@ -1640,7 +1640,7 @@
 \topline
 \hdstyle{Expression}  &   \hdstyle{Return type} &   \hdstyle{Requirement} \\ \capsep
 \tcode{a < b}   &
-convertible to \tcode{bool} &
+\tcode{decltype(a < b)} models \exposconceptx{boolean-testa\-ble}{boolean-testable} &
 \tcode{<} is a strict weak ordering relation\iref{alg.sorting}    \\
 \end{oldconcepttable}
 
@@ -1863,7 +1863,7 @@
 a value of type (possibly \keyword{const}) \tcode{std::nullptr_t}.
 
 \begin{oldconcepttable}{NullablePointer}{}{cpp17.nullablepointer}
-{lll}
+{lx{2in}l}
 \topline
 \lhdr{Expression} & \chdr{Return type} & \rhdr{Operational semantics} \\ \capsep
 \tcode{P u(np);}\br           &
@@ -1882,17 +1882,17 @@
   \ensures \tcode{t == nullptr}  \\ \rowsep
 
 \tcode{a != b}                &
-  contextually convertible to \tcode{bool}  &
+  \tcode{decltype(a != b)} models \exposconcept{boolean-testable} &
   \tcode{!(a == b)}           \\ \rowsep
 
 \tcode{a == np}               &
-  contextually convertible to \tcode{bool}  &
+  \tcode{decltype(a == np)} and \tcode{decltype(np == a)} each model \exposconcept{boolean-testable} &
   \tcode{a == P()}            \\
 \tcode{np == a}               &
                               &
                               \\ \rowsep
 \tcode{a != np}               &
-  contextually convertible to \tcode{bool}  &
+  \tcode{decltype(a != np)} and \tcode{decltype(np != a)} each model \exposconcept{boolean-testable} &
   \tcode{!(a == np)}          \\
 \tcode{np != a}               &
                               &

source/support.tex

@@ -5047,7 +5047,9 @@
   Otherwise, \tcode{strong_order(E, F)} if it is a well-formed expression.
 \item
   Otherwise, if the expressions \tcode{E == F} and \tcode{E < F}
-  are both well-formed and convertible to \tcode{bool},
+  are both well-formed and
+  each of \tcode{decltype(E == F)} and \tcode{decltype(E < F)} models
+  \exposconcept{boolean-testable},
 \begin{codeblock}
 E == F ? strong_ordering::equal :
 E < F  ? strong_ordering::less :
@@ -5079,7 +5081,9 @@
   Otherwise, \tcode{weak_order(E, F)} if it is a well-formed expression.
 \item
   Otherwise, if the expressions \tcode{E == F} and \tcode{E < F}
-  are both well-formed and convertible to \tcode{bool},
+  are both well-formed and
+  each of \tcode{decltype(E == F)} and \tcode{decltype(E < F)} models
+  \exposconcept{boolean-testable},
 \begin{codeblock}
 E == F ? weak_ordering::equivalent :
 E < F  ? weak_ordering::less :
@@ -5112,7 +5116,9 @@
 \item
   Otherwise, if the expressions
   \tcode{E == F}, \tcode{E < F}, and \tcode{F < E}
-  are all well-formed and convertible to \tcode{bool},
+  are all well-formed and
+  each of \tcode{decltype(E == F)} and \tcode{decltype(E < F)} models
+  \exposconcept{boolean-testable},
 \begin{codeblock}
 E == F ? partial_ordering::equivalent :
 E < F  ? partial_ordering::less :

source/threads.tex

@@ -30,8 +30,14 @@
 \pnum
 Throughout this Clause, the names of template parameters are used to express type
 requirements.
-If a template parameter is named \tcode{Predicate}, \tcode{operator()} applied to
-the template argument shall return a value that is convertible to \tcode{bool}.
+\tcode{Predicate} is a function object type\iref{function.objects}.
+Let \tcode{pred} denote an lvalue of type \tcode{Predicate}.
+Then the expression \tcode{pred()} shall be well-formed and
+the type \tcode{decltype(pred())} shall model
+\exposconcept{boolean-testable}\iref{concept.booleantestable}.
+The return value of \tcode{pred()}, converted to \tcode{bool},
+yields \tcode{true} if the corresponding test condition is satisfied, and
+\tcode{false} otherwise.
 If a template parameter is named \tcode{Clock},
 the corresponding template argument shall be a type \tcode{C}
 that meets the \oldconcept{Clock} requirements\iref{time.clock.req};

source/utilities.tex

@@ -1269,6 +1269,11 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\expects
+Each of \tcode{decltype(x.first == y.first)} and
+\tcode{decltype(x.second == y.second)} models \exposconcept{boolean-testable}.
+
 \pnum
 \returns
 \tcode{x.first == y.first \&\& x.second == y.second}.
@@ -2907,11 +2912,15 @@
 \mandates
 For all \tcode{i},
 where $0 \leq \tcode{i} < \tcode{sizeof...(TTypes)}$,
-\tcode{get<i>(t) == get<i>(u)} is a valid expression
-returning a type that is convertible to \tcode{bool}.
+\tcode{get<i>(t) == get<i>(u)} is a valid expression.
 \tcode{sizeof...(TTypes)} equals
 \tcode{tuple_size_v<UTuple>}.
 
+\pnum
+\expects
+For all \tcode{i}, \tcode{decltype(get<i>(t) == get<i>(u))} models
+\exposconcept{boolean-testable}.
+
 \pnum
 \returns
 \tcode{true} if \tcode{get<i>(t) == get<i>(u)} for all