<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62529>62529</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            -frelaxed-template-template-args: "error: pack expansion used as argument for non-pack parameter of alias template" with constrained alias template
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ecatmur
      </td>
    </tr>
</table>

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