-frelaxed-template-template-args: "error: pack expansion used as argument for non-pack parameter of alias template" with constrained alias template

[ecatmur]

Motivation:

#include <boost/hana/type.hpp>
template<class> requires(true) using U = int;
auto t = boost::hana::template_<U>(boost::hana::type_c<char>);

Reduced:

template<class> requires(true) using U = int;
template<template<typename ...> class, class> struct S {};
template<class T> S<U, T> f();
auto t = f<int>();

compiled with -frelaxed-template-template-args gives:

<source>:2:31: error: pack expansion used as argument for non-pack parameter of alias template
template<template<typename ...> class, class> struct S {};

(we need -frelaxed-template-template-args for other reasons)
Workaround: use std::enable_if_t to constrain alias template instead.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

CC @cor3ntin

Also some context: https://discourse.llvm.org/t/why-frelaxed-template-template-args-is-added-back/65727

[mizvekov]

I believe clang is correct in rejecting it, per CWG1430, which is still open, but has consensus on rejection.

The bug is not diagnosing it, which is an oversight in the pre-P0522 implementation of template template parameter matching, which is still used as a fast path in the post-P0522 implementation. This fast path is purposefully skipped when the argument template is constrained.