P3168R2 Give std::optional Range Support

source/support.tex

@@ -726,6 +726,7 @@
 #define @\defnlibxname{cpp_lib_not_fn}@                            202306L // freestanding, also in \libheader{functional}
 #define @\defnlibxname{cpp_lib_null_iterators}@                    201304L // freestanding, also in \libheader{iterator}
 #define @\defnlibxname{cpp_lib_optional}@                          202110L // also in \libheader{optional}
+#define @\defnlibxname{cpp_lib_optional_range_support}@            202406L // freestanding, also in \libheader{optional}
 #define @\defnlibxname{cpp_lib_out_ptr}@                           202311L // freestanding, also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_parallel_algorithm}@                201603L // also in \libheader{algorithm}, \libheader{numeric}
 #define @\defnlibxname{cpp_lib_polymorphic_allocator}@             201902L // also in \libheader{memory_resource}

source/utilities.tex

@@ -3168,6 +3168,11 @@
   template<class T>
     class optional;                                                     // partially freestanding
 
+  template<class T>
+    constexpr bool ranges::enable_view<optional<T>> = true;
+  template<class T>
+    constexpr auto format_kind<optional<T>> = range_format::disabled;
+
   template<class T>
     concept @\defexposconcept{is-derived-from-optional}@ = requires(const T& t) {       // \expos
       []<class U>(const optional<U>&){ }(t);
@@ -3248,7 +3253,9 @@
   template<class T>
   class optional {
   public:
-    using value_type = T;
+    using value_type     = T;
+    using iterator       = @\impdefnc@;              // see~\ref{optional.iterators}
+    using const_iterator = @\impdefnc@;              // see~\ref{optional.iterators}
 
     // \ref{optional.ctor}, constructors
     constexpr optional() noexcept;
@@ -3282,6 +3289,12 @@
     // \ref{optional.swap}, swap
     constexpr void swap(optional&) noexcept(@\seebelow@);
 
+    // \ref{optional.iterators}, iterator support
+    constexpr iterator begin() noexcept;
+    constexpr const_iterator begin() const noexcept;
+    constexpr iterator end() noexcept;
+    constexpr const_iterator end() const noexcept;
+
     // \ref{optional.observe}, observers
     constexpr const T* operator->() const noexcept;
     constexpr T* operator->() noexcept;
@@ -4001,6 +4014,59 @@
 the state of \tcode{*val} and \tcode{*rhs.val} is determined by the exception safety guarantee of \tcode{T}'s move constructor.
 \end{itemdescr}
 
+\rSec3[optional.iterators]{Iterator support}
+
+\indexlibrarymember{iterator}{optional}%
+\indexlibrarymember{const_iterator}{optional}%
+\begin{itemdecl}
+using iterator       = @\impdef@;
+using const_iterator = @\impdef@;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+These types
+model \libconcept{contiguous_iterator}\iref{iterator.concept.contiguous},
+meet the \oldconcept{RandomAccessIterator} requirements\iref{random.access.iterators}, and
+meet the requirements for constexpr iterators\iref{iterator.requirements.general},
+with value type \tcode{remove_cv_t<T>}.
+The reference type is \tcode{T\&} for \tcode{iterator} and
+\tcode{const T\&} for \tcode{const_iterator}.
+
+\pnum
+All requirements on container iterators\iref{container.reqmts} apply to
+\tcode{optional::iterator} and \tcode{optional::\linebreak{}const_iterator} as well.
+
+\pnum
+Any operation that initializes or destroys the contained value of an optional object invalidates all iterators into that object.
+\end{itemdescr}
+
+\indexlibrarymember{begin}{optional}%
+\begin{itemdecl}
+constexpr iterator begin() noexcept;
+constexpr const_iterator begin() const noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+If \tcode{has_value()} is \tcode{true},
+an iterator referring to the contained value.
+Otherwise, a past-the-end iterator value.
+\end{itemdescr}
+
+\indexlibrarymember{end}{optional}%
+\begin{itemdecl}
+constexpr iterator end() noexcept;
+constexpr const_iterator end() const noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{begin() + has_value()}.
+\end{itemdescr}
+
 \rSec3[optional.observe]{Observers}
 
 \indexlibrarymember{operator->}{optional}%