<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/147177>147177</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Cannot declare friend function template with unqualified name in C++20 mode without prior forward declaration
        </td>
    </tr>

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

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

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

<pre>
    This program
```c++
template <typename T>
class A {
    friend void f<>(A);
};

template <typename T>
void f(A<T>) {}

int main() {
    f(A<int>{});
}
```
is accepted in GCC and MSVC starting from C++20. But Clang keeps complaining:
```
error: no candidate function template was found for dependent friend function template specialization
    3 |     friend void f<>(A);
      | ^
```
Online demo: https://gcc.godbolt.org/z/vKjhY9WMv

It looks related to ADL changes for function templates lookup with explicitly-specified template arguments in C++20: https://en.cppreference.com/w/cpp/language/adl.html#Notes 
GCC and MSVC both allow unqualified names in template friend declaration `friend void f<>(A&);`, which is accepted if function template `f` is declared after class `A` body but before its instantiation.

</pre>
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