| Issue |
180131
|
| Summary |
Clang rejects valid program saying implicit instantiation of undefined primary template
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
ranaanoop
|
The following program is rejected by clang but accepted by both gcc and msvc. [Demo](https://godbolt.org/z/KKe9jT8E3)
```
#include <type_traits>
template <class...>
struct fn_ref_call;
template <class R, class... Args>
struct fn_ref_call<R (Args...)> {
static constexpr bool enable = true;
};
template <class... S>
struct fn_ref : private fn_ref_call<S...> {
private:
using base = fn_ref_call<S...>;
public:
template<class F>
requires (base::enable)
fn_ref(F* f){}
};
template<class F>
requires std::is_function_v<F>
fn_ref(F *) -> fn_ref<F>;
int fn(int) { return 3; }
int main() {
fn_ref f{&fn};
}
```
Clang says:
```
<source>:12:25: error: implicit instantiation of undefined template 'fn_ref_call<>'
12 | struct fn_ref : private fn_ref_call<S...> {
| ^
<source>:18:19: note: in instantiation of template class 'fn_ref<>' requested here
18 | requires (base::enable)
| ^
<source>:18:19: note: while substituting template arguments into constraint _expression_ here
18 | requires (base::enable)
| ^~~~~~~~~~~~
<source>:30:12: note: while checking constraint satisfaction for template '<deduction guide for fn_ref><int (int)>' required here
30 | fn_ref f{&fn};
| ^
<source>:30:12: note: while substituting deduced template arguments into function template '<deduction guide for fn_ref>' [with S = (no value), F = int (int)]
<source>:4:8: note: template is declared here
4 | struct fn_ref_call;
```
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