https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108169
Bug ID: 108169
Summary: class type template parameters are non-const in GCC
(differs from other compilers)
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: danakj at orodu dot net
Target Milestone: ---
Reproduces on GCC 10, 11, 12 and Trunk.
An auto template parameter's type is const in Clang and MSVC, but is considered
non-const in GCC. This does not occur with primitive types, but it does with
class types.
This is a C++20 bug, as C++20 introduced class type template parameters. With
-std=c++17, gcc states as such:
```
<source>:15:26: error: non-type template parameters of class type only
available with '-std=c++20' or '-std=gnu++20'
```
Source code:
```
#include <type_traits>
template <auto V>
struct S {
static constexpr bool is_const = std::is_const_v<decltype(V)>;
};
struct W {
constexpr W(int i) : i(i) {}
int i;
};
int main() {
static_assert(!S<1>::is_const); // pass
static_assert(!S<W(2)>::is_const); // fail
}
```
Godbolt: https://godbolt.org/z/WEbPcjzcK
