Merge 2019-07 LWG Motion 2

P1355R2 Exposing a narrow contract for ceil2

Fixes #3006.

Author: zygoloid

source/numerics.tex

@@ -1312,7 +1312,7 @@
   template<class T>
     constexpr bool ispow2(T x) noexcept;
   template<class T>
-    constexpr T ceil2(T x) noexcept;
+    constexpr T ceil2(T x);
   template<class T>
     constexpr T floor2(T x) noexcept;
   template<class T>
@@ -1382,21 +1382,34 @@
 \indexlibrary{\idxcode{ceil2}}%
 \begin{itemdecl}
 template<class T>
-  constexpr T ceil2(T x) noexcept;
+  constexpr T ceil2(T x);
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+Let $N$ be the smallest power of 2 greater than or equal to \tcode{x}.
+
+\pnum
+\constraints
+\tcode{T} is an unsigned integer type\iref{basic.fundamental}.
+
+\pnum
+\expects
+$N$ is representable as a value of type \tcode{T}.
+
 \pnum
 \returns
-The minimal value \tcode{y} such that
-\tcode{ispow2(y)} is \tcode{true} and \tcode{y >= x};
-if \tcode{y} is not representable as a value of type \tcode{T},
-the result is an unspecified value.
+$N$.
+
+\pnum
+\throws
+Nothing.
 
 \pnum
 \remarks
-This function shall not participate in overload resolution
-unless \tcode{T} is an unsigned integer type\iref{basic.fundamental}.
+A function call expression
+that violates the precondition in the \expects element
+is not a core constant expression\iref{expr.const}.
 \end{itemdescr}
 
 \indexlibrary{\idxcode{floor2}}%