Allow typename in a template template parameter (original) (raw)

ISO/IEC JTC1 SC22 WG21
N4051
Richard Smith
2014-05-26

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:

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

type-parameter:

…

class ...opt identifieropt

class identifieropt = id-expression

typename ...opt identifieropt

typename identifieropt = id-expression

template < template-parameter-list > class ...opt identifieropt>

template < template-parameter-list > class identifieropt = 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 identifieropt

class type-parameter-key identifieropt = id-expression

typename ...opt identifieropt

typename identifieropt = id-expression

template < template-parameter-list > class type-parameter-key ...opt identifieropt>

template < template-parameter-list > class type-parameter-key identifieropt = 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 atemplate-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 declaredwith class or typenamewithout template) or_template-name_ (if declared with template) in the scope of the template declaration. [Note: …]