Why is the following template declaration ill-formed? [duplicate]

Question

Why is the following declaration invalid?

template<template<typename> typename T>
struct S {};

I would assume this was valid since the following is valid:

template<template<typename> class T>
struct S {};

and what I can read from the standard in [gram.temp] it appears to be valid, but gcc gives me the following output:

prog.cpp:4:38: error: expected 'class' before 'T'
 template<template<typename> typename T>
                                  ^

Answer 1

Basically, "because the standard says so." C++11 14.1/1 lists the syntax for type-parameter:

type-parameter:
  class ..._optidentifier_opt
  class identifier_opt= type-id
  typename ..._optidentifier_opt
  typename identifier_opt= type-id
  template < template-parameter-list > class ..._optidentifier_opt
  template < template-parameter-list > class identifier_opt= id-expression

As you can see, only class is allowed for template template parameters.

My guess for the rationale is that any type can be used as a type argument, including non-class types. But before C++11, there was no such thing as a "non-class type template"—the only type templates were class templates.

This has changed with C++11's alias templates, but the template template parameter syntax has obviously not kept up. Nevertheless, in a post-C++11 (actually post-C++14) draft N4296, this restriction is actually lifted and template <class> typename is valid template template parameter syntax.