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Document number: N4051

Date: 2014-05-26

Allow typename in a template template parameter

by Richard Smith

Excerpt:

Since the introduction of alias templates, C++ has had type templates that are not class templates, and in particular now has template template arguments that are not class templates. However, the syntax for template template parameters still requires the class keyword be used:

template<typename T> struct A {};
template<typename T> using B = int;

template<template<typename> class X> struct C;
C<A> ca; // ok
C<B> cb; // ok, not a class template
template<template<typename> typename X> struct D; // error, cannot use typename here

... This difference is artificial and is a common surprise. Removing it would make the language simpler.