Fix long ambiguity in conversion ctor

Refs: gh-26

Signed-off-by: Matthias Kretz <kretz@kde.org>

P0214/wording-datapar.tex

@@ -50,8 +50,9 @@
 \begin{itemdescr}
   \pnum\remarks This constructor shall not participate in overload resolution unless
   \begin{itemize}
-    \item \type{abi_type} equals \fixedsizescoped{}\code{<size()>}
-    \item and every possible value of \type U can be represented with type \valuetype.
+    \item \type{abi_type} equals \fixedsizescoped{}\code{<size()>},
+    \item and every possible value of \type U can be represented with type \valuetype,
+    \item and, if both \type U and \valuetype are integral, the integer conversion rank \citep[(4.15)]{N4618} of \valuetype is greater than the integer conversion rank of \type U.
   \end{itemize}
   \pnum\effects Constructs an object where the $i$-th element equals \code{static_cast<T>(x[i])} \foralli.
 \end{itemdescr}

wg21.bib

@@ -177,3 +177,11 @@ @misc{N4582
     url = {http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4582.pdf},
     year = {2016}
 }
+@misc{N4618,
+    shorthand = {N4618},
+    editor = {Smith, Richard},
+    organization = {ISO/IEC JTC1/SC22/WG21},
+    title = {{Working Draft, Standard for Programming Language \CC{}}},
+    url = {http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4618.pdf},
+    year = {2016}
+}