format user-defined type get a compile error after 8.0.1

[subenle-nreal]

test code is listed below。It works well with version 8.0.1，but will get a compile error with newer version even the newest version 10.0.0。when build with 10.0.0，I uncomment two formmater in the code, just as the document list。

#include <fmt/ostream.h>
#include <iostream>
class date {
  int year_, month_, day_;
public:
  date(int year, int month, int day): year_(year), month_(month), day_(day) {}

  friend std::ostream& operator<<(std::ostream& os, const date& d) {
    return os << d.year_ << '-' << d.month_ << '-' << d.day_;
  }
};

class MyVector3f {
   public:
    float x = 1;
    float y = 2;
    float z = 3;

   public:
    MyVector3f() {}
    template <typename LocalType>
    MyVector3f(const LocalType& v) {
        this->x = v.x();
        this->y = v.y();
        this->z = v.z();
    }

    template <typename LocalType>
    operator LocalType() const {
        LocalType v;
        v.x() = this->x;
        v.y() = this->y;
        v.z() = this->z;
        return v;
    }

    template <typename OStream>
 	friend OStream& operator<<(OStream& os, const MyVector3f& v) {
    return os << "["
              << " x:" << v.x << ", y:" << v.y << ", z:" << v.z << "]";
}
};

//  When build with 10.0.0, please uncomment the two lines below.
// template <> struct fmt::formatter<date> : ostream_formatter {};
// template <> struct fmt::formatter<MyVector3f> : ostream_formatter{};


int main()
{
	std::string s = fmt::format("The date is {}", date(2012, 12, 9));
	std::cout << s << std::endl;

	std::string vs = fmt::format("vector3f is {}", MyVector3f());
	std::cout << vs << std::endl;
}

[vitaut]

Repro: https://godbolt.org/z/WWod8GdhP.

The problem is that MyVector3f has an implicit conversion operator that tries to convert to anything but fails if it doesn't have members x(), etc. which interferes with built-in {fmt}'s formatters. I would recommend constraining it, e.g.

    template <typename LocalType, size_t = sizeof(LocalType().x())>
    operator LocalType() const {
        LocalType v;
        v.x() = this->x;
        v.y() = this->y;
        v.z() = this->z;
        return v;
    }

[subenle-nreal]

Thanks, it really works.

[vitaut]

Fixed in 606f85f.