[Bug c++/85250] New: Builtin operator overload resolution fails to consider user-defined conversion template

[rustamabd at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85250

            Bug ID: 85250
           Summary: Builtin operator overload resolution fails to consider
                    user-defined conversion template
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rustamabd at gmail dot com
  Target Milestone: ---

Consider the following contrived example.

-----------------------------------------------------------
#include <type_traits>

struct A {
    template<typename T, typename
std::enable_if<std::is_same<T,int>::value,int>::type = 0>
    operator T() { return static_cast<T>(1); }
};

int main() {
    int x = 1;
    A a;
    return x + a;
}
-----------------------------------------------------------
Output from g++:
error: no match for 'operator+' (operand types are 'int' and 'A')
  return x + a;
         ~~^~~
-----------------------------------------------------------
Compiles fine in clang.
-----------------------------------------------------------

A lookup on + should find the built-in int operator+(int&,int) as the only
viable candidate (see [over.match.oper]/6).
That candidate is viable because A can be converted to int via a user-defined
conversion.

If operator int() isn't a template, G++ is able to compile the code.
It's just the conversion function *template* that fails to be considered.