P2281R1 Clarifying range adaptor objects

Also fixes LWG3509 and LWG3510.

Author: jensmaurer

source/lib-intro.tex

@@ -771,15 +771,34 @@
 \pnum
 All instances of a specific customization point object type shall
 be equal\iref{concepts.equality}.
+The effects of invoking different instances
+of a specific customization point object type on the same arguments
+are equivalent.
 
 \pnum
-The type \tcode{T} of a customization point object shall model
-\tcode{\libconcept{invocable}<const T\&, Args...>}\iref{concept.invocable}
+The type \tcode{T} of a customization point object,
+ignoring \grammarterm{cv-qualifier}s, shall model
+\tcode{\libconcept{invocable}<T\&, Args...>},
+\tcode{\libconcept{invocable}<const T\&, Args...>},
+\tcode{\libconcept{invocable}<T, Args...>}, and
+\tcode{\libconcept{invocable}<const T, Args...>}\iref{concept.invocable}
 when the types in \tcode{Args...} meet the requirements specified in that
 customization point object's definition. When the types of \tcode{Args...} do
 not meet the customization point object's requirements, \tcode{T} shall not have
 a function call operator that participates in overload resolution.
 
+\pnum
+For a given customization point object \tcode{o},
+let \tcode{p} be a variable initialized as if by \tcode{auto p = o;}.
+Then for any sequence of arguments \tcode{args...},
+the following expressions have effects equivalent to \tcode{o(args...)}:
+\begin{itemize}
+\item \tcode{p(args...)}
+\item \tcode{as_const(p)(args...)}
+\item \tcode{std::move(p)(args...)}
+\item \tcode{std::move(as_const(p))(args...)}
+\end{itemize}
+
 \pnum
 Each customization point object type constrains its return type to model a
 particular concept.

source/ranges.tex

@@ -3055,20 +3055,30 @@
 a range adaptor closure object \tcode{C} and an expression \tcode{R} such that
 \tcode{decltype((R))} models \libconcept{viewable_range}, the following
 expressions are equivalent and yield a \libconcept{view}:
-
 \begin{codeblock}
 C(R)
 R | C
 \end{codeblock}
-
 Given an additional range adaptor closure object \tcode{D},
-the expression \tcode{C | D} is well-formed and produces another range adaptor
-closure object such that the following two expressions are equivalent:
-
-\begin{codeblock}
-R | C | D
-R | (C | D)
-\end{codeblock}
+the expression \tcode{C | D} produces another range adaptor
+closure object \tcode{E}.
+\tcode{E} is a perfect forwarding call wrapper\iref{func.require}
+with the following properties:
+\begin{itemize}
+\item
+Its target object is an object \tcode{d} of type \tcode{decay_t<decltype((D))>}
+direct-non-list-initialized with \tcode{D}.
+\item
+It has one bound argument entity,
+an object \tcode{c} of type \tcode{decay_t<decltype((C))>}
+direct-non-list-initialized with \tcode{C}.
+\item
+Its call pattern is \tcode{d(c(arg))},
+where \tcode{arg} is the argument used in
+a function call expression of \tcode{E}.
+\end{itemize}
+The expression \tcode{C | D} is well-formed if and only if
+the initializations of the state entities of \tcode{E} are all well-formed.
 
 \pnum
 A \term{range adaptor object} is a
@@ -3081,17 +3091,28 @@
 then it is a range adaptor closure object.
 
 \pnum
-If a range adaptor object accepts more than one argument,
-then the following expressions are equivalent:
-
-\begin{codeblock}
-@\placeholdernc{adaptor}@(range, args...)
-@\placeholdernc{adaptor}@(args...)(range)
-range | @\placeholdernc{adaptor}@(args...)
-\end{codeblock}
-
-In this case, \tcode{\placeholdernc{adaptor}(args...)} is a range adaptor
-closure object.
+If a range adaptor object \tcode{adaptor} accepts more than one argument,
+then let \tcode{range} be an expression
+such that \tcode{decltype((range))} models \libconcept{viewable_range},
+let \tcode{args...} be arguments
+such that \tcode{adaptor(range, args...)} is a well-formed expression
+as specified in the rest of subclause\iref{range.adaptors}, and
+let \tcode{BoundArgs} be a pack
+that denotes \tcode{decay_t<decltype((args))>...}.
+The expression \tcode{adaptor(args...)} produces a range adaptor closure object \tcode{f}
+that is a perfect forwarding call wrapper with the following properties:
+\begin{itemize}
+\item
+Its target object is a copy of \tcode{adaptor}.
+\item
+Its bound argument entities \tcode{bound_args} consist of objects of types \tcode{BoundArgs...} direct-non-list-initialized with \tcode{std::forward<decltype((args))>(args)...}, respectively.
+\item
+Its call pattern is \tcode{adaptor(r, bound_args...)},
+where \tcode{r} is the argument used in a function call expression of \tcode{f}.
+\end{itemize}
+The expression \tcode{adaptor(args...)} is well-formed if and only if
+the initialization of the bound argument entities of the result,
+as specified above, are all well-formed.
 
 \rSec2[range.semi.wrap]{Semiregular wrapper}