<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/125913>125913</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Alias template CTAD is erroneously allowed in C++17 mode
        </td>
    </tr>

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

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

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

<pre>
    Alias template CTAD is a C++20 feature ( https://en.cppreference.com/w/cpp/compiler_support/20#cpp_deduction_guides_201907L ) recently introduced in clang v19. Seems it's being allowed in C++17 mode with no diagnostic ( https://godbolt.org/z/x7zdxnrqx ). Though a (almost correct) diagnostic is issued with the `-Wpre-c++17-compat` flag ( https://godbolt.org/z/acKsG3dfj ).

Testcase is replicated here
```
template<typename T, typename TT>
struct S
{
    T t;
    TT tt;
};

template<typename T, typename TT>
S(T, TT) -> S<T, TT>;

template<typename T>
using S_int = S<T, int>;

int main()
{
 (void) S_int{1, 2};
}
```
</pre>
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