https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117392
Bug ID: 117392
Summary: template argument deduction problem with CNTTP
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hanicka at hanicka dot net
Target Milestone: ---
Link to code and comparison with clang:
https://compiler-explorer.com/z/zaqjYzfs4
In this a bit convoluted code I found while working on `P2781R4
std::constant_wrapper`
When inside operators::operator- I tried to instantiate constant_wrapper<42u>
(with unsigned int)
```c++
template <typename T> struct fixed_value {
T data;
using type = T;
constexpr fixed_value(type v) noexcept: data(v) { }
};
template <fixed_value Value, typename AdlType = decltype(Value)::type>
struct constant_wrapper;
```
It fails because decltype(Value) is `unsigned int` and not
`fixed_value<unsigned int>`
If I replace `= decltype(Value)::type` with `= void` the link / code is
working.
```c++
template <typename T> struct fixed_value {
T data;
using type = T;
constexpr fixed_value(type v) noexcept: data(v) { }
};
// if I use following line it will work
// it seems when it's trying to instantiate `constant_wrapper`
// it's using `Value` as parametr (of unsigned int) and not argument of type
`fixed_value<unsigned int>`
// and because of that the `decltype(Value)::type` fails
// template <fixed_value Value, typename AdlType = void>
template <fixed_value Value, typename AdlType = decltype(Value)::type>
struct constant_wrapper;
struct operators {
template <typename L, typename R>
friend constexpr auto operator-(L, R) noexcept
-> constant_wrapper<(L::value - R::value)>
{ return {}; }
};
template <fixed_value Value, typename AdlType> struct constant_wrapper:
operators {
static constexpr auto & value = Value.data;
};
constexpr auto x = constant_wrapper<42u>{} - constant_wrapper<12>{};
```
