<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/130992>130992</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] 20.1.0 fails to compile variadic partial template specialization
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    The following minimum repro used to compile successfully and work correctly with Clang 19.1.4, but fails to compile with Clang 20.1.0:
```
#include <iostream>
#include <utility>

template <typename> struct s;
template <typename T> constexpr std::size_t size = s<T>::value;
// list
template <template <typename...> typename L, typename... U>
struct s<L<U...>> { static constexpr std::size_t value = sizeof...(U); };
// sequence
template <typename T, template <typename, T...> typename L, T... U>
struct s<L<T, U...>> { static constexpr std::size_t value = sizeof...(U); };

#define TEST(N) \
  std::cout << "[" #N "]: " << size<std::make_index_sequence<N>> << std::endl;

int main() {
  TEST(0); TEST(1); TEST(2);
  TEST(3); TEST(4); TEST(5);
}
```

Compiler output in Clang 20.1.0:
```
$ clang++ -Wall -std=c++23 sz.cpp -o sz
../sz.cpp:5:52: error: implicit instantiation of undefined template 's<std::integer_sequence<unsigned long>>'
    5 | template <typename T> constexpr std::size_t size = s<T>::value;
      | ^
../sz.cpp:17:3: note: in instantiation of variable template specialization 'size<std::integer_sequence<unsigned long>>' requested here
   17 |   TEST(0);
      |   ^
../sz.cpp:14:32: note: expanded from macro 'TEST'
   14 |   std::cout << "[" #N "]: " << size<std::make_index_sequence<N>> << std::endl;
      | ^
../sz.cpp:4:28: note: template is declared here
    4 | template <typename> struct s;
      |                            ^
```
</pre>
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