P3913R1 Optimize for std::optional in range adaptors

Fixes PL-011 22.5 (C++26 CD).

Fixes #8481

Author: steve-downey

source/ranges.tex

@@ -5898,6 +5898,10 @@
 except that the evaluations of \tcode{E} and \tcode{F}
 are indeterminately sequenced.
 
+\item
+Otherwise, if \tcode{T} is a specialization of
+\tcode{optional} and \tcode{T} models \tcode{view},
+then \tcode{(static_cast<D>(F) == D() ? ((void)E, T()) : \placeholdernc{decay-copy}(E))}.
 \item
 Otherwise, if \tcode{T} models
 \libconcept{random_access_range} and \libconcept{sized_range}
@@ -6381,6 +6385,10 @@
 except that the evaluations of \tcode{E} and \tcode{F}
 are indeterminately sequenced.
 
+\item
+Otherwise, if \tcode{T} is a specialization of \tcode{optional}
+and \tcode{T} models \tcode{view},
+then \tcode{(static_cast<D>(F) == D() ? \placeholdernc{decay-copy}(E) : ((void)E, T()))}.
 \item
 Otherwise, if \tcode{T} models
 \libconcept{random_access_range} and \libconcept{sized_range}
@@ -9909,6 +9917,9 @@
   If the type of \tcode{E} is
   a (possibly cv-qualified) specialization of \tcode{reverse_view},
   then \tcode{E.base()}.
+\item
+  Otherwise, if \tcode{E} is specialization of \tcode{optional} and \tcode{E}
+  models \tcode{view}, then \tcode{\placeholdernc{decay-copy}(E)}.
 \item
   Otherwise, if the type of \tcode{E} is \cv{} \tcode{subrange<reverse_iterator<I>, reverse_iterator<I>, K>}
   for some iterator type \tcode{I} and
@@ -10061,6 +10072,9 @@
 if \tcode{U} denotes \tcode{empty_view<X>}
 for some type \tcode{X}, then \tcode{auto(views::empty<const X>)}.
 \item
+Otherwise, if \tcode{U} denotes \tcode{optional<X\&>}
+for some type \tcode{X}, then \tcode{optional<const X\&>(E)}.
+\item
 Otherwise,
 if \tcode{U} denotes \tcode{span<X, Extent>}
 for some type \tcode{X} and some extent \tcode{Extent},