P3865R3 Class template argument deduction (CTAD) for type template template

Author: eisenwave

source/declarations.tex

@@ -1444,16 +1444,24 @@
 if any, shall be non-dependent and
 the \grammarterm{template-name} or \grammarterm{splice-specifier}
 shall designate a deducible template.
-A \defnadj{deducible}{template} is either a class template or
-is an alias template whose \grammarterm{defining-type-id} is of the form
+A \defnadj{deducible}{template} is
+\begin{itemize}
+\item
+a class template,
+\item
+a type template template parameter, or
+\item
+an alias template \tcode{A} whose \grammarterm{defining-type-id} is of the form
 
 \begin{ncsimplebnf}
 \opt{\keyword{typename}} \opt{nested-name-specifier} \opt{\keyword{template}} simple-template-id
 \end{ncsimplebnf}
 
 where the \grammarterm{nested-name-specifier} (if any) is non-dependent and
 the \grammarterm{template-name} of the \grammarterm{simple-template-id}
-names a deducible template.
+names a deducible template
+other than a type template template parameter of \tcode{A}.
+\end{itemize}
 \begin{note}
 An injected-class-name is never interpreted as a \grammarterm{template-name}
 in contexts where class template argument deduction would be performed\iref{temp.local}.

source/overloading.tex

@@ -1431,6 +1431,20 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+When resolving a placeholder for a deduced class type
+where the \grammarterm{template-name}
+designates a type template template parameter \tcode{P},
+the type template template argument for \tcode{P}
+shall be a deducible template.
+Let \tcode{A} be an alias template
+whose template parameter list is that of \tcode{P} and
+whose \grammarterm{defining-type-id} is a \grammarterm{simple-template-id}
+whose \grammarterm{template-name} designates the type template template argument and
+whose \grammarterm{template-argument-list} is the template argument list of \tcode{P}.
+\tcode{A} is then used instead of the original \grammarterm{template-name}
+to resolve the placeholder.
+
 \pnum
 When resolving a placeholder for a deduced class type\iref{dcl.type.simple}
 where
@@ -1688,6 +1702,25 @@
 \indextext{overloading!argument lists|)}%
 \indextext{overloading!candidate functions|)}
 
+\pnum
+\begin{example}
+\begin{codeblock}
+template<typename ... Ts>
+struct Y {
+  Y();
+  Y(Ts ...);
+};
+template<template<typename T = char> class X>
+void f() {
+  X x;          // OK, deduces \tcode{Y<char>}
+  X x0{};       // OK, deduces \tcode{Y<char>}
+  X x1{1};      // OK, deduces \tcode{Y<int>}
+  X x2{1, 2};   // error: cannot deduce \tcode{X<T>} from \tcode{Y<int, int>}
+}
+template void f<Y>();
+\end{codeblock}
+\end{example}
+
 \rSec2[over.match.viable]{Viable functions}%
 \indextext{overloading!resolution!viable functions|(}
 

source/templates.tex

@@ -5660,7 +5660,9 @@
 is dependent if
 \begin{itemize}
 \item
-it has a dependent initializer, or
+it has a dependent initializer,
+\item
+it refers to a type template template parameter, or
 \item
 it refers to an alias template
 that is a member of the current instantiation and