CWG2600 Type dependency of placeholder types

Author: burblebee

source/expressions.tex

@@ -5139,7 +5139,9 @@
 \end{bnf}
 
 \pnum
-If a placeholder type\iref{dcl.spec.auto} appears in the
+If a placeholder type\iref{dcl.spec.auto} or
+a placeholder for a deduced class type\iref{dcl.type.class.deduct}
+appears in the
 \grammarterm{type-specifier-seq} of a \grammarterm{new-type-id} or
 \grammarterm{type-id} of a \grammarterm{new-expression},
 the allocated type is deduced as follows:

source/templates.tex

@@ -5016,6 +5016,45 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+An initializer is dependent if
+any constituent expression\iref{intro.execution} of the initializer
+is type-dependent.
+A placeholder type\iref{dcl.spec.auto.general} is dependent if
+it designates a type deduced from a dependent initializer.
+
+\pnum
+A placeholder for a deduced class type\iref{dcl.type.class.deduct}
+is dependent if
+\begin{itemize}
+\item
+it has a dependent initializer, or
+\item
+it refers to an alias template
+that is a member of the current instantiation and
+whose \grammarterm{defining-type-id} is dependent after
+class template argument deduction\iref{over.match.class.deduct}
+and substitution\iref{temp.alias}.
+\end{itemize}
+
+\pnum
+\begin{example}
+\begin{codeblock}
+template<class T, class V>
+struct S { S(T); };
+
+template<class U>
+struct A {
+  template<class T> using X = S<T, U>;
+  template<class T> using Y = S<T, int>;
+  void f() {
+    new X(1);                   // dependent
+    new Y(1);                   // not dependent
+  }
+};
+\end{codeblock}
+\end{example}
+
 \pnum
 A type is dependent if it is
 \begin{itemize}
@@ -5038,6 +5077,10 @@
 \item
 a function type whose exception specification is value-dependent,
 \item
+denoted by a dependent placeholder type,
+\item
+denoted by a dependent placeholder for a deduced class type,
+\item
 denoted by a \grammarterm{simple-template-id}
 in which either the template name is a template parameter or any of the
 template arguments is a dependent type or an expression that is type-dependent