Merge 2018-11 LWG Motion 20

P1248R1 Remove CommonReference requirement from StrictWeakOrdering (a.k.a Fixing Relations)

Fixes #2426

Author: zygoloid

source/concepts.tex

@@ -1080,35 +1080,9 @@
 template<class R, class T, class U>
   concept Relation =
     Predicate<R, T, T> && Predicate<R, U, U> &&
-    CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
-    Predicate<R,
-      common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>,
-      common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>> &&
     Predicate<R, T, U> && Predicate<R, U, T>;
 \end{itemdecl}
 
-\begin{itemdescr}
-\pnum
-Let
-\begin{itemize}
-\item \tcode{r} be an expression such that \tcode{decltype((r))} is \tcode{R},
-\item \tcode{t} be an expression such that \tcode{decltype((t))} is \tcode{T},
-\item \tcode{u} be an expression such that \tcode{decltype((u))} is \tcode{U},
-  and
-\item \tcode{C} be
-  \begin{codeblock}
-  common_reference_t<const remove_reference_t<T>&,
-                     const remove_reference_t<U>&>
-  \end{codeblock}
-\end{itemize}
-\tcode{\libconcept{Relation}<R, T, U>} is satisfied only if
-
-\begin{itemize}
-\item \tcode{bool(r(t, u)) == bool(r(C(t), C(u))).}
-\item \tcode{bool(r(u, t)) == bool(r(C(u), C(t))).}
-\end{itemize}
-\end{itemdescr}
-
 \rSec2[concept.strictweakorder]{Concept \libconcept{StrictWeakOrder}}
 
 \indexlibrary{\idxcode{Relation}}%