Failure to match alias template against template template parameter

[ecatmur]

template<template<class T> class> struct A {};
template<class T> struct Q {};
template<class T> using R = Q<T>;
int f(A<R>);
int g(A<Q> a) { return f(a); }

GCC accepts (since 4.9.0), Clang rejects with:

<source>:5:24: error: no matching function for call to 'f'
int g(A<Q> a) { return f(a); }
                       ^
<source>:4:5: note: candidate function not viable: no known conversion from 'A<template Q>' to 'A<template R>' for 1st argument
int f(A<R>);
    ^

-frelaxed-template-template-args doesn't seem to help in any recent major.

[llvmbot]

@llvm/issue-subscribers-c-1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

gcc is the only implementation that accepts it: https://godbolt.org/z/v9Trvovza

Can you explain why you expect this to be valid?

[ecatmur]

I think this is CWG 1286. It looks like we haven't got a resolution in yet, but the intent seems clear that it should be accepted.

[jakub-homola]

Stumbled upon the same issue today.

For anyone looking for a workaround, this is one of the ways that worked for me (assuming that Q does not have weird specializations, testing only for one, char):

#include <type_traits>

template<template<class T> class> struct A {};
template<class T> struct Q {};
template<class T> using R = Q<T>;

template<template<class> class U>
int f(A<U>)
{
    static_assert(std::is_same_v<U<char>,R<char>>);
    return 42;
}

int g(A<Q> a) { return f(a); }

or a series of if constexprs if f needs to be "overloaded" for multiple Us.