[17 regression][C++20] templated friend not working if the concept use contains the enclosing class

[zero9178]

Recently upgraded to Clang 17 which lead to code that previously worked in Clang 16 to suddently stop compiling. I have reduced it to the following snippet:

template <class T, class U>
concept Test = true;

class UnwindFrame
{
    template <Test<UnwindFrame> F>
    friend void unwindStack();

    UnwindFrame();
};

template <Test<UnwindFrame> F>
void unwindStack()
{
    auto u = UnwindFrame();
}

int main()
{
    unwindStack<int>();
}

https://godbolt.org/z/1vcqeGnG1

When using Clang 17 or newer the friend declaration seemingly has no effect causing a compilation failure due to the use of a private constructor:

<source>:16:14: error: calling a private constructor of class 'UnwindFrame'
   16 |     auto u = UnwindFrame();
      |              ^
<source>:21:5: note: in instantiation of function template specialization 'unwindStack<int>' requested here
   21 |     unwindStack<int>();
      |     ^
<source>:10:5: note: implicitly declared private here
   10 |     UnwindFrame();
      |     ^

This used to work in Clang 16 and is valid C++20 I believe.

While reducing, I noticed the following important properties for this to be reproducible: unwindStack HAS to be restricted by a concept and the concept HAS to be templated, with a template parameter being the enclosing class UnwindFrame in this case.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Markus Böck (zero9178)

Recently upgraded to Clang 17 which lead to code that previously worked in Clang 16 to suddently stop compiling. I have reduced it to the following snippet: ```cpp template <class T, class U> concept Test = true;

class UnwindFrame
{
template <Test<UnwindFrame> F>
friend void unwindStack();

UnwindFrame();

};

template <Test<UnwindFrame> F>
void unwindStack()
{
auto u = UnwindFrame();
}

int main()
{
unwindStack<int>();
}

https://godbolt.org/z/1vcqeGnG1

When using Clang 17 or newer the `friend` declaration seemingly has no effect causing a compilation failure due to the use of a private constructor:

<source>:16:14: error: calling a private constructor of class 'UnwindFrame'
16 | auto u = UnwindFrame();
| ^
<source>:21:5: note: in instantiation of function template specialization 'unwindStack<int>' requested here
21 | unwindStack<int>();
| ^
<source>:10:5: note: implicitly declared private here
10 | UnwindFrame();
| ^

This used to work in Clang 16 and is valid C++20 I believe.

While reducing, I noticed the following important properties for this to be reproducible: `unwindStack` HAS to be restricted by a concept and the concept HAS to be templated, with a template parameter being the enclosing class `UnwindFrame` in this case. 
</details>

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Markus Böck (zero9178)

Recently upgraded to Clang 17 which lead to code that previously worked in Clang 16 to suddently stop compiling. I have reduced it to the following snippet: ```cpp template <class T, class U> concept Test = true;

class UnwindFrame
{
template <Test<UnwindFrame> F>
friend void unwindStack();

UnwindFrame();

};

template <Test<UnwindFrame> F>
void unwindStack()
{
auto u = UnwindFrame();
}

int main()
{
unwindStack<int>();
}

https://godbolt.org/z/1vcqeGnG1

When using Clang 17 or newer the `friend` declaration seemingly has no effect causing a compilation failure due to the use of a private constructor:

<source>:16:14: error: calling a private constructor of class 'UnwindFrame'
16 | auto u = UnwindFrame();
| ^
<source>:21:5: note: in instantiation of function template specialization 'unwindStack<int>' requested here
21 | unwindStack<int>();
| ^
<source>:10:5: note: implicitly declared private here
10 | UnwindFrame();
| ^

This used to work in Clang 16 and is valid C++20 I believe.

While reducing, I noticed the following important properties for this to be reproducible: `unwindStack` HAS to be restricted by a concept and the concept HAS to be templated, with a template parameter being the enclosing class `UnwindFrame` in this case. 
</details>

[shafik]

https://godbolt.org/z/Y5Pz3W7d6

CC @erichkeane @cor3ntin

[joka921]

I recently ran into a similar issue:

• Also uses a templated friend function where the concept uses the enclosing class
• Same compiler versions (working in Clang 16 and all recent versions of GCC, broken in Clang 17 and the current trunk)
• Different error (call is ambiguous).

#include <concepts>

struct S {
    template <std::same_as<S> T>
    friend void f(const T&);
};

template <std::same_as<S> T>
void f(const T& t) {}

int main() {
    f(S{});
}

Gives

<source>:13:5: error: call to 'f' is ambiguous
   13 |     f(S{});
      |     ^
<source>:6:17: note: candidate function [with T = S]
    6 |     friend void f(const T&);
      |                 ^
<source>:10:6: note: candidate function [with T = S]
   10 | void f(const T& t) {}

Please let me know if this is similar enough to also be discussed in this issue, or if I should open a new one.

[antangelo]

I believe the issue is with checking if the constraint uses any enclosing template parameters for [temp.friend]p9. If a containing record is used as argument, we assume that the constraint depends on enclosing template parameters without checking if the record is a template. In the above reproducers UnwindFrame and S are not template classes so this should not apply.

I'm not the most well versed with the standard though, so I'd appreciate if someone more experienced can give their input before I send a patch for this.