| Issue |
71595
|
| Summary |
[17 regression][C++20] templated friend not working if the concept use contains the enclosing friend
|
| Labels |
clang:frontend,
regression
|
| Assignees |
|
| Reporter |
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.
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