Alias template produces seemingly bogus "template template argument has different template parameters" error

[Quuxplusone]

Bugzilla Link	37157
Version	trunk
OS	All
CC	@DougGregor

Extended Description

I (and also GCC 7.0-and-later) claim that this program is legal C++.

template<template<class> class X> struct A {};
template<template<class...> class Y> using B = A<Y>;
template<class> struct S;
B<S> b;

Clang, on the other hand, bails out at line 2:

prog.cc:2:50: error: template template argument has different template parameters than its corresponding template template parameter
template<template<class...> class Y> using B = A;
^
prog.cc:2:19: note: template type parameter pack does not match template type parameter in template argument
template<template<class...> class Y> using B = A;
^
prog.cc:1:19: note: previous template type parameter declared here
template<template class X> struct A {};
^

Clang becomes happy if I change it from 'using B = A' to 'struct B {}', and then sad again if I change it to 'struct B : A {}'.

[llvmbot]

Looks like -frelaxed-template-template-args enables the fix for this (off by default). See the commit message for r290792 for more info. If we're going to keep this off by default, it would be nice to have better diagnostics for cases like this.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Arthur O'Dwyer (Quuxplusone)

| | | | --- | --- | | Bugzilla Link | [37157](https://llvm.org/bz37157) | | Version | trunk | | OS | All | | CC | @DougGregor |

Extended Description

I (and also GCC 7.0-and-later) claim that this program is legal C++.

template<template<class> class X> struct A {};
template<template<class...> class Y> using B = A<Y>;
template<class> struct S;
B<S> b;

Clang, on the other hand, bails out at line 2:

prog.cc:2:50: error: template template argument has different template parameters than its corresponding template template parameter
template<template<class...> class Y> using B = A<Y>;
^
prog.cc:2:19: note: template type parameter pack does not match template type parameter in template argument
template<template<class...> class Y> using B = A<Y>;
^
prog.cc:1:19: note: previous template type parameter declared here
template<template<class> class X> struct A {};
^

Clang becomes happy if I change it from 'using B = A<Y>' to 'struct B {}', and then sad again if I change it to 'struct B : A<Y> {}'.