https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119398
Bug ID: 119398
Summary: partial ordering of nested class template
specializations fails with NTTP of nested class type
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: eric.niebler at gmail dot com
Target Milestone: ---
The following is valid C++20:
template <class T>
struct outer {
struct value_type {
T value{};
constexpr value_type() = default;
constexpr value_type(T v) : value(v) {}
};
template <auto, value_type...>
struct inner;
template <auto _>
struct inner<_, value_type{}, value_type{}> {};
template <auto _, value_type I>
struct inner<_, I, value_type{}> {};
template <auto _, value_type J>
struct inner<_, value_type{}, J> {};
template <auto _, value_type I, value_type J>
struct inner<_, I, J> {};
};
int main() {
[[maybe_unused]]
outer<int>::inner<0, 0, 1> i{};
}
GCC rejects it with:
<source>: In function 'int main()':
<source>:28:30: error: ambiguous template instantiation for 'struct
outer<int>::inner<0, outer<int>::value_type{0}, outer<int>::value_type{1}>'
28 | outer<int>::inner<0, 0, 1> i{};
| ^
It fails on trunk. I've also tested as far back as gcc-12, and it has never
worked.
https://godbolt.org/z/vfP5Y9hG9
