https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126442
Bug ID: 126442
Summary: binary operator deduction fails to match SFINAE
conversion function
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: phiren at gmail dot com
Target Milestone: ---
Minimal-ish reproduction:
```
template<typename> struct is_int {};
template<> struct is_int<int> { using type = int; };
struct T {
template<typename U, typename = typename is_int<U>::type>
operator U() { return {}; }
};
int main ()
{
T a;
0+a; // error: no match for 'operator+' (operand types are 'int' and 'T')
a+a; // error: no match for 'operator+' (operand types are 'T' and 'T')
}
```
As far as I can tell, there is nothing in the c++ spec saying this shouldn't
work.
The first `0+a` expression actually works in clang (but not the second)
Both expressions failed in every single gcc/msvc version I checked on godbolt,
all the from latest trunk all the way back to gcc 5
