Template argument deduction fails when passing enum - c++

Question

I'm trying to insert an enum parameter into a constexpr function. I have done this in the past and it always worked... except in this case. This case is only special in that I'm calling a factory function first. But apparently C++ doesn't see through this. What can be done?
Those are my errors:
<source>: In function 'constexpr auto operator+(some_enum)':
<source>:30:28: error: no matching function for call to 'signal<1, state>(std::nullptr_t)'
30 | return signal<1, state>(nullptr);
| ~~~~~~~~~~~~~~~~^~~~~~~~~
<source>:23:37: note: candidate: 'template<bool set, some_enum S, class ... Ts> constexpr signal_str<sizeof... (Ts)> signal(Ts ...)'
23 | constexpr signal_str<sizeof...(Ts)> signal(Ts... Args)
| ^~~~~~
<source>:23:37: note: template argument deduction/substitution failed:
<source>:30:28: error: 'state' is not a constant expression
30 | return signal<1, state>(nullptr);
| ~~~~~~~~~~~~~~~~^~~~~~~~~
<source>:30:28: note: in template argument for type 'some_enum'
<source>:28:16: error: invalid return type 'auto' of 'constexpr' function 'constexpr auto operator+(some_enum)'
28 | constexpr auto operator+(some_enum state)
| ^~~~~~~~
Compiler returned: 1
This is my code:
#include <array>
#include <cstdint>
#include <iostream>
typedef void* TaskType_t;
enum some_enum
{
SOME_STATE = 1,
};
template <size_t N>
struct signal_str
{
uint32_t val_;
std::array<TaskType_t, N> tasks_;
};
template <bool set, some_enum S, typename... Ts>
constexpr signal_str<sizeof...(Ts)> signal(Ts... Args)
{
return signal_str<sizeof...(Ts)>{S, {Args...}}.val_;
}
constexpr auto operator+(some_enum state)
{
return signal<1, state>(nullptr);
}
int main()
{
static_assert(+SOME_STATE);
}
I'm using C++17 on xtensa-gcc 8.2.0 but it's the same with gcc 11 (LIVE DEMO).
EDIT: This problem is different from "Why is const variable necessary for template specialization over constants" because enums are already constants. To showcase this the following DOES actually compile:
#include <array>
#include <cstdint>
#include <iostream>
typedef void* TaskType_t;
enum some_enum
{
SOME_STATE = 1,
};
template <size_t N>
struct signal_str
{
uint32_t val_;
std::array<TaskType_t, N> tasks_;
};
constexpr auto operator+(some_enum state)
{
return signal_str<1>{state, nullptr}.val_;
}
int main()
{
static_assert(+SOME_STATE);
}
DEMO
So the problem IMHO is not the enum..