Merge 2019-07 LWG Motion 12

P1614R2 Adding <=> to the Library

Fixes #3016.

Author: zygoloid

source/algorithms.tex

@@ -2830,19 +2830,17 @@
                                 Proj1 proj1 = {}, Proj2 proj2 = {});
   }
 
-  // \ref{alg.3way}, three-way comparison algorithms
-  template<class T, class U>
-    constexpr auto compare_3way(const T& a, const U& b);
+  // \ref{alg.three.way}, three-way comparison algorithms
   template<class InputIterator1, class InputIterator2, class Cmp>
     constexpr auto
-      lexicographical_compare_3way(InputIterator1 b1, InputIterator1 e1,
-                                   InputIterator2 b2, InputIterator2 e2,
-                                   Cmp comp)
+      lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
+                                        InputIterator2 b2, InputIterator2 e2,
+                                        Cmp comp)
         -> common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>;
   template<class InputIterator1, class InputIterator2>
     constexpr auto
-      lexicographical_compare_3way(InputIterator1 b1, InputIterator1 e1,
-                                   InputIterator2 b2, InputIterator2 e2);
+      lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
+                                        InputIterator2 b2, InputIterator2 e2);
 
   // \ref{alg.permutation.generators}, permutations
   template<class BidirectionalIterator>
@@ -8367,46 +8365,15 @@
 \end{note}
 \end{itemdescr}
 
-\rSec2[alg.3way]{Three-way comparison algorithms}
+\rSec2[alg.three.way]{Three-way comparison algorithms}
 
-\indexlibrary{\idxcode{compare_3way}}%
-\begin{itemdecl}
-template<class T, class U> constexpr auto compare_3way(const T& a, const U& b);
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Compares two values and produces a result
-of the strongest applicable comparison category type:
-\begin{itemize}
-\item
-  Returns \tcode{a <=> b} if that expression is well-formed.
-\item
-  Otherwise, if the expressions \tcode{a == b} and \tcode{a < b}
-  are each well-formed and convertible to \tcode{bool},
-  returns \tcode{strong_ordering::equal}
-  when \tcode{a == b} is \tcode{true},
-  otherwise returns \tcode{strong_ordering::less}
-  when \tcode{a < b} is \tcode{true},
-  and otherwise returns \tcode{strong_ordering::greater}.
-\item
-  Otherwise, if the expression \tcode{a == b}
-  is well-formed and convertible to \tcode{bool},
-  returns \tcode{strong_equality::equal} when \tcode{a == b} is \tcode{true},
-  and otherwise returns \tcode{strong_equality::nonequal}.
-\item
-  Otherwise, the function is defined as deleted.
-\end{itemize}
-\end{itemdescr}
-
-\indexlibrary{\idxcode{lexicographical_compare_3way}}%
+\indexlibrary{\idxcode{lexicographical_compare_three_way}}%
 \begin{itemdecl}
 template<class InputIterator1, class InputIterator2, class Cmp>
   constexpr auto
-    lexicographical_compare_3way(InputIterator1 b1, InputIterator1 e1,
-                                 InputIterator2 b2, InputIterator2 e2,
-                                 Cmp comp)
+    lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
+                                      InputIterator2 b2, InputIterator2 e2,
+                                      Cmp comp)
       -> common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>;
 \end{itemdecl}
 
@@ -8431,23 +8398,20 @@
 \end{codeblock}
 \end{itemdescr}
 
-\indexlibrary{\idxcode{lexicographical_compare_3way}}%
+\indexlibrary{\idxcode{lexicographical_compare_three_way}}%
 \begin{itemdecl}
 template<class InputIterator1, class InputIterator2>
   constexpr auto
-    lexicographical_compare_3way(InputIterator1 b1, InputIterator1 e1,
-                                 InputIterator2 b2, InputIterator2 e2);
+    lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
+                                      InputIterator2 b2, InputIterator2 e2);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \effects
 Equivalent to:
 \begin{codeblock}
-return lexicographical_compare_3way(b1, e1, b2, e2,
-                                    [](const auto& t, const auto& u) {
-                                      return compare_3way(t, u);
-                                    });
+return lexicographical_compare_three_way(b1, e1, b2, e2, compare_three_way());
 \end{codeblock}
 \end{itemdescr}