220 30694 <CANu6V4U0GDA5Zoi7Ri-fvBp+eM05gXYawQghu69MGTkdS3=91Q@mail.gmail.com> article
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From: "'Johannes Schaub' via ISO C++ Standard - Future Proposals" <std-proposals@isocpp.org>
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Subject: Re: template template aliases?
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2017-01-30 18:24 GMT+01:00 Eric Niebler <eric.niebler@gmail.com>:
> Crazy thought.
>
> Why not permit template template aliases?
>
>   template <class A>
>   template <class B>
>   using Foo = pair<A, B>;
>
> Foo<A> is a unary template such that Foo<A><B> is pair<A,B>.
>
> Why?
>
> Higher order template metaprogramming sometimes requires passing a template
> to a template. For example:
>
>   // with C++17 fold expressions
>   template <template <class Ts> class Pred, class...Ts>
>   constexpr bool all_of = (... && Pred<Ts>::value);
>
>   static_assert(all_of<std::is_scalar, int, short, float>);
>
> Q: How would you use all_of to check that all types in a pack are
> convertible to bool?
> A: With a "bind"-like template template alias:
>
>   template <template<class...> class Fn, class...As>
>   template <class...Bs>
>   using bind_back = Fn<Bs..., As...>;
>
>   template <template<class...> class Fn>
>   template <class T>
>   using unary = Fn<T>;
>
>   static_assert(all_of<unary<bind_back<std::is_convertible, bool>>, int,
> short, float>);
>
> What would this language feature break?
>

Some note about the syntax. I think you need to extend it so that
"template" can be put after the angle brackets aswell?

    template<typename T>
    struct A { template<typename A> template<typename B> using X =
void (A::*)(B); };

    template<typename T>
    void f() {
       typename T::template X<string> template<size_t> p = &string::resize;
    }

    int main() { f<A>(); }

Note the second "template" disambiguator.

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