Clang rejects valid program saying implicit instantiation of undefined primary template

The following program is rejected by clang but accepted by both gcc and msvc. [Demo](https://godbolt.org/z/KKe9jT8E3)

```c++
#include

template
struct fn_ref_call;

template
struct fn_ref_call {
static constexpr bool enable = true;
};

template
struct fn_ref : private fn_ref_call {
private:
using base = fn_ref_call;

public:
template
requires (base::enable)
fn_ref(F* f){}
};

template
requires std::is_function_v
fn_ref(F *) -> fn_ref;

int fn(int) { return 3; }

int main() {
fn_ref f{&fn};
}
```
Clang says:
```
:12:25: error: implicit instantiation of undefined template 'fn_ref_call<>'
12 | struct fn_ref : private fn_ref_call {
| ^
:18:19: note: in instantiation of template class 'fn_ref<>' requested here
18 | requires (base::enable)
| ^
:18:19: note: while substituting template arguments into constraint expression here
18 | requires (base::enable)
| ^~~~~~~~~~~~
:30:12: note: while checking constraint satisfaction for template '' required here
30 | fn_ref f{&fn};
| ^
:30:12: note: while substituting deduced template arguments into function template '' [with S = (no value), F = int (int)]
:4:8: note: template is declared here
4 | struct fn_ref_call;
```

Start by reproducing the provided C++ program with Clang and compare its behavior with GCC and MSVC using the linked Godbolt demo. Trace constraint satisfaction and template instantiation around the `fn_ref` deduction guide and `fn_ref_call<>`; done means Clang accepts the valid program and a regression test covers this case.

Written by the indexing model from the issue text.