Allow typename in a template template parameter

Rationale

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

The relevant grammar rules are in 14.1 (temp.param) paragraph 1:

type-parameter:

…

class ..._opt identifier_opt

class identifier_opt = id-expression

typename ..._opt identifier_opt

typename identifier_opt = id-expression

template < template-parameter-list > class ..._opt identifier_opt>

template < template-parameter-list > class identifier_opt = id-expression

Note that typename is permitted for non-template type-parameters, but not for template type-parameters. This difference is artificial and is a common surprise. Removing it would make the language simpler.

Proposed wording

Change in 14.1 (temp.param) paragraph 1:

type-parameter:

…

class type-parameter-key ..._opt identifier_opt

class type-parameter-key identifier_opt = id-expression

typename ..._opt identifier_opt

typename identifier_opt = id-expression

template < template-parameter-list > class type-parameter-key ..._opt identifier_opt>

template < template-parameter-list > class type-parameter-key identifier_opt = id-expression

type-parameter-key:

class

typename

Change in 14.1 (temp.param) paragraph 2:

There is no semantic difference between class and typename in a template-parameter type-parameter-key. typename followed by an unqualified-id names a template type parameter. typename followed by a qualified-id denotes the type in a non-type [Footnote: …] parameter-declaration. … [Note: A template parameterargument may be a class template or alias template. For example, …]

Change in 14.1 (temp.param) paragraph 3:

A type-parameter whose identifier does not follow an ellipsis defines its identifier to be a typedef-name (if declared with class or typenamewithout template) or template-name (if declared with template) in the scope of the template declaration. [Note: …]