LWG4308 std::optional<T&>::iterator can't be a contiguous iterator for some T

Author: burblebee

source/utilities.tex

@@ -4469,8 +4469,9 @@
   template<class T>
   class optional<T&> {
   public:
-    using value_type     = T;
-    using iterator       = @\impdefnc@;              // see~\ref{optional.ref.iterators}
+    using value_type = T;
+    using iterator   = @\impdefnc@;  // present only if \tcode{T} is an object type other than an array
+                                                // of unknown bound; see~\ref{optional.ref.iterators}
 
   public:
     // \ref{optional.ref.ctor}, constructors
@@ -4503,8 +4504,8 @@
     constexpr void swap(optional& rhs) noexcept;
 
     // \ref{optional.ref.iterators}, iterator support
-    constexpr iterator begin() const noexcept;
-    constexpr iterator end() const noexcept;
+    constexpr auto begin() const noexcept;
+    constexpr auto end() const noexcept;
 
     // \ref{optional.ref.observe}, observers
     constexpr T*       operator->() const noexcept;
@@ -4778,7 +4779,8 @@
 \rSec3[optional.ref.iterators]{Iterator support}
 
 \begin{itemdecl}
-using iterator = @\impdef@;
+using iterator = @\impdefnc@;        // present only if \tcode{T} is an object type other than an array
+                                                // of unknown bound
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4796,22 +4798,31 @@
 \end{itemdescr}
 
 \begin{itemdecl}
-constexpr iterator begin() const noexcept;
+constexpr auto begin() const noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\constraints
+\tcode{T} is an object type other than an array of unknown bound.
+
 \pnum
 \returns
-  If \tcode{has_value()} is \tcode{true},
-  an iterator referring to \tcode{*\exposid{val}}.
-  Otherwise, a past-the-end iterator value.
+  An object \tcode{i} of type \tcode{iterator}, such that
+  \tcode{i} is an iterator referring to \tcode{*\exposid{val}}
+  if \tcode{has_value()} is \tcode{true}, and
+  a past-the-end iterator value otherwise.
 \end{itemdescr}
 
 \begin{itemdecl}
-constexpr iterator end() const noexcept;
+constexpr auto end() const noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\constraints
+\tcode{T} is an object type other than an array of unknown bound.
+
 \pnum
 \returns
 \tcode{begin() + has_value()}.