Merge 2021-06 LWG Motion 12

P1989R2 Range constructor for std::string_view 2: Constrain Harder

Author: tkoeppe

source/strings.tex

@@ -4091,6 +4091,8 @@
     constexpr basic_string_view(const charT* str, size_type len);
     template<class It, class End>
       constexpr basic_string_view(It begin, End end);
+    template<class R>
+      constexpr basic_string_view(R&& r);
 
     // \ref{string.view.iterators}, iterator support
     constexpr const_iterator begin() const noexcept;
@@ -4179,9 +4181,11 @@
     size_type size_;            // \expos
   };
 
-  // \ref{string.view.deduct}, deduction guide
+  // \ref{string.view.deduct}, deduction guides
   template<class It, class End>
     basic_string_view(It, End) -> basic_string_view<iter_value_t<It>>;
+  template<class R>
+    basic_string_view(R&&) -> basic_string_view<ranges::range_value_t<R>>;
 }
 \end{codeblock}
 
@@ -4281,6 +4285,45 @@
 initializes \tcode{size_} with \tcode{end - begin}.
 \end{itemdescr}
 
+\indexlibraryctor{basic_string_view}%
+\begin{itemdecl}
+template<class R>
+  constexpr basic_string_view(R&& r);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+Let \tcode{d} be an lvalue of type \tcode{remove_cvref_t<R>}.
+
+\pnum
+\constraints
+\begin{itemize}
+\item
+\tcode{R} models
+\tcode{ranges::\libconcept{contiguous_range}} and \tcode{ranges::\libconcept{sized_range}},
+\item
+\tcode{is_same_v<ranges::range_value_t<R>, charT>} is \tcode{true},
+\item
+\tcode{is_convertible_v<R, const charT*>} is \tcode{false},
+\item
+\tcode{d.operator ::std::basic_string_view<charT, traits>()}
+is not a valid expression, and
+\item
+if the \grammarterm{qualified-id} \tcode{remove_reference_t<R>::traits_type}
+is valid and denotes a type,
+\tcode{is_same_v<remove_reference_t<R>::traits_type, traits>} is \tcode{true}.
+\end{itemize}
+
+\pnum
+\effects
+Initializes \tcode{data_} with \tcode{ranges::data(r)} and
+\tcode{size_} with \tcode{ranges::size(r)}.
+
+\pnum
+\throws
+Any exception thrown by \tcode{ranges::data(r)} and \tcode{ranges::size(r)}.
+\end{itemdescr}
+
 \rSec3[string.view.iterators]{Iterator support}
 
 \indexlibrarymember{const_iterator}{basic_string_view}%
@@ -4956,7 +4999,7 @@
 Otherwise, returns \tcode{npos}.
 \end{itemdescr}
 
-\rSec2[string.view.deduct]{Deduction guide}
+\rSec2[string.view.deduct]{Deduction guides}
 
 \begin{itemdecl}
 template<class It, class End>
@@ -4972,6 +5015,17 @@
 \end{itemize}
 \end{itemdescr}
 
+\begin{itemdecl}
+template<class R>
+  basic_string_view(R&&) -> basic_string_view<ranges::range_value_t<R>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\constraints
+\tcode{R} satisfies \tcode{ranges::\libconcept{contiguous_range}}.
+\end{itemdescr}
+
 \rSec2[string.view.comparison]{Non-member comparison functions}
 
 \pnum