static constexpr class members are required to have a definition

[kivadiu]

The following program does not compile with clang++ 5.0.2. With g++ 7.3.0, it does not compile in debug mode but compiles with -O2. The error is that the class static constexpr int i is required to have an address because fmt::format takes its arguments by const reference. However I do not understand why g++ work in release mode! Would it be possible that fmt::format takes its arguments by universal references &&? Would that improve this situation?

#include <iostream>
#include <fmt/format.h>

struct Foo {
  static constexpr auto i = 42;
  void operator()() const { std::cout << fmt::format("{}\n", i); }
};

int main() {
  Foo{}();
  return 0;
}

[vitaut]

I don't see any error with clang 5.0: https://godbolt.org/g/DrbUZe

[kivadiu]

The problem is not at compile time but at link time, you can see it if you click on 11010 in godbolt:

/tmp/example-852a44.o: In function `Foo::operator()() const':
<source>:6: undefined reference to `Foo::i'

[vitaut]

Would it be possible that fmt::format takes its arguments by universal references &&? Would that improve this situation?

No, it wouldn't, e.g.

struct Foo {
  static constexpr auto i = 42;
};

template <typename T>
void f(T&& i) {}

int main() {
  f(Foo::i);
}

gives

Undefined symbols for architecture x86_64:
  "Foo::i", referenced from:
      _main in test1-c21676.o

According to [basic.def.odr]:

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement; no diagnostic required.

This requirement was relaxed in C++17, so your example should compile there. On C++14 and earlier you should provide an out-of-class definition of your variable.

[kivadiu]

In fact with c++17, it fails both with g++ 8.1.0 and clang 6.0.0 with a much less easy to understand message.
https://godbolt.org/g/dAnHxG

[vitaut]

That's because Compiler Explorer doesn't link with the fmt library. Your example compiles when using header-only mode: https://godbolt.org/g/DhyUfd

[kivadiu]

OK, thanks, I did not know that godbolt could not link with libraries.