NB RU-1 (C++17 CD) Why must empty or fully-initialized const objects …

…be initialized?

Also fixes CWG 253.

source/declarators.tex

@@ -2594,8 +2594,31 @@
 no initialization is performed.
 \end{itemize}
 
+A class type \tcode{T} is \defn{const-default-constructible} if
+default-initialization of \tcode{T} would invoke
+a user-provided constructor of \tcode{T} (not inherited from a base class)
+or if
+
+\begin{itemize}
+\item
+each direct non-variant non-static data member \tcode{M} of \tcode{T}
+has a default member initializer
+or, if \tcode{M} is of class type \tcode{X} (or array thereof),
+\tcode{X} is const-default-constructible,
+\item
+if \tcode{T} is a union with at least one non-static data member,
+exactly one variant member has a default member initializer,
+\item
+if \tcode{T} is a not a union,
+for each anonymous union member with at least one non-static data member (if any),
+exactly one non-static data member has a default member initializer, and
+\item
+each potentially constructed base class of \tcode{T} is const-default-constructible.
+\end{itemize}
+
 If a program calls for the default-initialization of an object of a
-const-qualified type \tcode{T}, \tcode{T} shall be a class type with a user-provided default constructor.
+const-qualified type \tcode{T},
+\tcode{T} shall be a const-default-constructible class type or array thereof.
 
 \pnum
 \indextext{value-initialization}%