Failing to compare two template template types with fixed number of arguments-CodePudding

I have a SFINAE template version of is_same for comparing template template arguments. It generally works as expected on both templates with fixed and variadic number of template parameters:

namespace eld::util::traits
{
        template<template<typename...> class ATT, template<typename...> class BTT>
        struct is_same_tt : std::false_type
        {
        };

        template<template<typename...> class TT>
        struct is_same_tt<TT, TT> : std::true_type
        {
        };
}

template<typename AT, typename BT, typename CT>
struct fixed_args_number
{
};

template<typename...>
struct variadic_args
{
};

static_assert(eld::util::traits::is_same_tt<fixed_args_number, fixed_args_number>(), "");
static_assert(eld::util::traits::is_same_tt<variadic_args, variadic_args>(), "");

But when I compare a template template with a fixed number of template parameters to a member template template, I get false, though true, when parameters are variadic.

template<template<typename...> class TT>
struct designated
{
    template<typename... T>
    using tt_type = TT<T...>;
};

// Does not compile
//static_assert(
//    eld::util::traits::is_same_tt<fixed_args_number, designated<fixed_args_number>::tt_type>(),
//    "");
static_assert(eld::util::traits::is_same_tt<variadic_args, designated<variadic_args>::tt_type>(),
              "");

What is the reason? (I only checked it with MinGW64)

The initial problem is to find in a type list a designated class that satisfies the condition for the equality of template template types.

CodePudding user response：

Neither should work: GCC has a bug of helpfulness that it considers an alias template to be the same as the underlying template if they’re sufficiently similar. The fixed/variadic mismatch, while still usable in practice, is enough for it to revert to conforming behavior.