template template reportedly undefined

[uohmak4fvpqe]

clang trunk (-std=c++20) fails on the following code :

#include<tuple>
#include<type_traits>

template<class>struct type_unpack;
    template<template<class/*...*/>class Pack,class T>struct type_unpack<Pack<T>>:std::type_identity<T>{};

static type_unpack<std::tuple<int>> t[[maybe_unused]];
int main(){
    return 0;
}

source>:7:37: error: implicit instantiation of undefined template 'type_unpack<std::tuple<int>>'

the fix I found is to add the .... GCC trunk compiles with or without the ellipsis.
Tested on godbolt

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[dwblaikie]

Is it clear (some standards wording would be helpful) that this is a Clang rejects-valid, and not a GCC accepts-invalid?

I would be surprised if std::tuple<int> would match against template<class> class X - I'd expect to need the ... to match the template argument list there, but I don't know for sure/don't have spec wording to back that up.

[royjacobson]

Is this another case of P0522? (#42305)

It compiles with the -frelaxed-template-template-args flag.

[uohmak4fvpqe]

Is this another case of P0522? (#42305)

It compiles with the -frelaxed-template-template-args flag.

Indeed it does, thanks

[uohmak4fvpqe]

Is it clear (some standards wording would be helpful) that this is a Clang rejects-valid, and not a GCC accepts-invalid?

I would be surprised if std::tuple<int> would match against template<class> class X - I'd expect to need the ... to match the template argument list there, but I don't know for sure/don't have spec wording to back that up.

cppreference does mention template<template<typename> class X>. I'm not surprised that putting an ellipsis in there matches any list of type parameters obvisously, but I expect to be able to constrain more, e.g thing like template<template<class,class,class> class X> that would only match things of arity 3 like std::tuple<char,int,void>

which is how it seems to work with -frelaxed-template-template-args