<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/60777>60777</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [15->17trunk regression] Parameter packs in requires-clauses in deduction guides are considered non-constant
        </td>
    </tr>

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

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

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

<pre>
    ```c++
template<typename... Ts>
constexpr bool True() { return true; }

template<typename T>
class A {
public:
    template<typename... Ts>
    A(T val, Ts... tail)
 requires (True<Ts...>())
    {}
};

int main() {
    A b(42);
}
```
Works on GCC, MSVC, and Clang 15.0.0. But on 17.0.0-26662ac010ef50e65e2774eab84f325aa09360fe, it fails with
```
<source>:9:15: error: substitution into constraint expression resulted in a non-constant expression
    requires (True<Ts...>())
 ^~~~~~~~~~~~~
<source>:14:7: note: while checking constraint satisfaction for template '<deduction guide for A><int>' required here
 A b(42);
      ^
<source>:14:7: note: in instantiation of function template specialization '<deduction guide for A><int>' requested here
<source>:9:15: note: non-literal type '<dependent type>' cannot be used in a constant expression
    requires (True<Ts...>())
 ^
<source>:14:7: error: no viable constructor or deduction guide for deduction of template arguments of 'A'
    A b(42);
 ^
<source>:8:5: note: candidate template ignored: failed template argument deduction
    A(T val, Ts... tail)
    ^
<source>:5:7: note: candidate template ignored: could not match 'A<T>' against 'int'
class A {
      ^
```
https://godbolt.org/z/sh7TrPGE1

I also found an ICE-on-invalid that I suspect may have the same root cause and/or same solution: https://godbolt.org/z/7qGTddsYh If it doesn't, feel free to ignore it; if this bug is fixed but the ICE remains, I'll file a new bug.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJyslktv4zYQgH8NfRnYkCjLsg8-OIq9yKHAAjVa9EiRI4kNTXr5yD4O_e3F0K8km91tgBoGbA2Hw2-elAhBDxZxzeo7Vt9PRIqj8-uNkcK7SefU1zVbFKevZPyOvsU9KzYRD0cjIrKqjV-PaMUBZ7MZ7AOrticN6WyI-OXooXPOwN4nZHzJ-ApYcwceY_IWIkmrO2DN_WnXD63D_mbZiBBgQ3ZOgmPqjJasOu8GAPg1H2ltGF_u4UkYxlvYB9KIQhvGV2cdj5-S9hiAFDNrm9XISnbmpgqQga6ONPesunvulLYRDkLbWxSegUDH-HLOydx119XUJQOnxz-dfwzgLHxoW-L-7fc_8q-wCloj7ABlPStmxQzuUiS9sqHHKV8sFlzIoiywrwtc1MibZo6iW877itdCFKtqUfRItnSEXmgT4LOO45sUrGqDS14ihaLarFi1KWtWbQC9d57-hNSFqGOK2lnQNjrIJeEFBYIKA0OgJY8hmYgKtAUB1tlp1hMvtG6hekdKWL3959nnTfByzqpNQ7zWUbls4POoDYIcUT5qOzyHDiLq0AuZPeqdv1YZMN6wqlWo0mlxSFphVtnkY1ptYwZsLvwKRvR4Bn0r_ZA_rN7-R2xNQc5h0yIzuB76ZE88V9BwRKmF0d9OOu_kxhBfgP-wCC5QlE2jI3phgPrweuARrUIbs_B8gBTWuggdQgqXavgfK-EXYbyWrXXwpEVHJZAzn2R0HpyHt6J0k7n-FmThh3RAGwNJGW825PVPe_2HgEtWbV5EVAqrtKJTrsfpwTqPilapaVF9T3IDfcfw-0n51a-r7-dY0iWjSBcOIsrxFJOq3Z8zLwZBpUtiqrdLsL6b869b4uVEGmM8BroE-I7x3eBU50ycOT8wvvvG-C6Mzd5__LAtnw_lBxAmOOhdsgqEhYd2O3V2qu2TMFpBHEWEBwiJ-obov8IonhDiiBDoVvLORZAiBaT5y_jO-dNCcCZPPnL_V2TNpw97pcJfIzz0NHqVw2AZbyJlpkc00HtEiO4cU9CRLk3dQxx1gC4NoAP0-gsq6FLMdA_tFjzSfRPIygPjjTHQ02gTYPEz7ZpN1LpSq2olJrguF81ivuTlqp6Ma7nqsC5UU9flvCzVnFcLNS-KJS5LXCFfTvSaF7wqeFmXDV_V9axvZF3KspfLRVWXFbJ5QaebmTFPB_J1okNIuF4UTdNMjOjQhPzWwTnR5EXGOb2E-DXtmXZpCGxeGB1iuFmJOpr8ulLWU1Ztyyb6ZB_B43CZD_U9fBReHDCih6OQj4FmyWVYTKWhbGXZq44OIPyp67VCmtDP76JJ8mb9KpE6jqmbSXdgfEeA55_p0bu_UUbGd9mtwPguu_1vAAAA___q7NvU">