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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/94679] link time error: undefined reference to
 std::projected<...>::operator *() const
Date: Tue, 21 Apr 2020 08:41:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94679

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

template<typename T>
struct projected
{
  T operator*() const;
};

struct IMove
{
  template<typename T>
    decltype(auto)
    operator()(T&& t) const
    {
      return *t;
    }
};

IMove iter_move;

template<typename T>
concept fooable =3D requires (T t) { iter_move(t); };

template<typename T>
requires fooable<projected<T>>
void func(T)
{
}

int main()
{
  int i =3D 0;
  func(i);
}


g++ -std=3Dc++20  use.cc -coverage -fkeep-inline-functions
/usr/bin/ld: /tmp/ccS2FlMR.o: in function `decltype(auto)
IMove::operator()<projected<int>&>(projected<int>&) const':
/tmp/use.cc:13: undefined reference to `projected<int>::operator*() const'
collect2: error: ld returned 1 exit status=