alias template is not a class template?

Question

I am trying to write a type trait to detect if a type has a T::type of certain type. I am using code from this answer. For reference this is the part of the code I am using:

// See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf.
template <typename...>
using void_t = void;

// Primary template handles all types not supporting the operation.
template <typename, template <typename> class, typename = void_t<>>
struct detect : std::false_type {};

// Specialization recognizes/validates only types supporting the archetype.
template <typename T, template <typename> class Op>
struct detect<T, Op, void_t<Op<T>>> : std::true_type {};

I started simple with a trait to detect the T::type :

template <typename T>
using has_type_t = typename T::type;

template <typename T>
using has_type = detect<T, has_type_t>;

This works as expected, but when I ask for the actual type of T::type also I get errors that I do not understand:

template <typename X>
struct has_X_type_helper {
    template <typename T>
    using type = typename std::enable_if_t<std::is_same_v< typename T::type, X>,int>;
};

template <typename T,typename X>
using has_X_type = detect<T,has_X_type_helper<X>::type>;

GCC:

<source>:49:55: error: type/value mismatch at argument 2 in template parameter list for 'template<class, template<class> class<template-parameter-1-2>, class> struct detect'
   49 | using has_X_type = detect<T,has_X_type_helper<X>::type>;
      |                                                       ^
<source>:49:55: note:   expected a class template, got 'has_X_type_helper<X>::type'

and Clang is even more confusing for me

<source>:49:29: error: template argument for template template parameter must be a class template or type alias template
using has_X_type = detect<T,has_X_type_helper<X>::type>;
                            ^

Is has_X_type_helper<X>::type not a type alias template ? What is wrong in my code?

@ godbolt

Piotr Skotnicki Piotr Skotnicki · Accepted Answer · 2020-06-09T08:09:07

You need to indicate that the nested thing is a template:

template <typename T, typename X>
using has_X_type = detect<T, has_X_type_helper<X>::template type>;
//                                               ~~~~~~~~~^

alias template is not a class template?

1 Answers