CWG1670 auto as conversion-type-id

Also a late resolution of NB FI 4 (C++14 CD).

Author: eisenwave

source/declarations.tex

@@ -1859,7 +1859,8 @@
 \pnum
 A placeholder type can appear in
 the \grammarterm{decl-specifier-seq} or \grammarterm{type-specifier-seq}
-in the declared return type of a function declarator that declares a function;
+in the declared return type of a function declarator
+that declares a function other than a conversion function\iref{class.conv.fct};
 the return type of the function is
 deduced from non-discarded \tcode{return} statements, if any, in the body
 of the function\iref{stmt.if}.