https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114884
Bug ID: 114884
Summary: GCC rejects valid deduction using deduction guide
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jlame646 at gmail dot com
Target Milestone: ---
The following valid program afaik is rejected by gcc but accepted by clang and
msvc: https://godbolt.org/z/85sjenzrn
```
#include <cassert>
#include <iostream>
#include <array>
#include <vector>
#include <concepts>
#include <type_traits>
template<typename C>
class Vec
{
C elems;
public:
Vec(const std::initializer_list<C>& init)
{
}
};
template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
template<typename C>
Vec(C) -> Vec<C>;
int main()
{
Vec v0({1, 4, 7});
}
```
GCC says
```
<source>:20:93: error: template argument 2 is invalid
20 | template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
|
^
<source>:20:94: error: template argument 2 is invalid
20 | template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
|
^~
<source>:20:94: error: template argument 2 is invalid
<source>:20:94: error: template argument 2 is invalid
<source>:20:70: error: invalid template-id
20 | template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
|
^~~
<source>:20:75: error: template argument 1 is invalid
20 | template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
|
^~~~~
<source>:20:75: error: template argument 1 is invalid
<source>:20:75: error: template argument 1 is invalid
<source>:20:75: error: template argument 1 is invalid
<source>:20:66: error: invalid template-id
20 | template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
| ^~~
<source>:20:96: error: missing template arguments before ';' token
20 | template<typename T> Vec(const std::initializer_list<T>& vec) ->
Vec<std::array<T, vec.size()>>;
|
^
<source>:20:96: error: expected '>' before ';' token
<source>:20:96: error: trailing return type 'Vec<...auto...>' of deduction
guide is not a specialization of 'Vec<C>'
<source>: In function 'int main()':
<source>:26:9: error: 'Vec<...auto...> v0' has incomplete type
26 | Vec v0({1, 4, 7});
| ^~
Compiler returned: 1
```
