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From: "Konstantin.Sadov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/59579] New: Defaulted copy constructor of template class is instantiated even when not called (probably bug 57153)
Date: Sun, 22 Dec 2013 17:30:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59579

            Bug ID: 59579
           Summary: Defaulted copy constructor of template class is
                    instantiated even when not called (probably bug 57153)
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Konstantin.Sadov at gmail dot com

The following code

struct A{A()=default;A(A&)=default;};
template<typename B>struct C:B{C()=default;
#if 1
C(const C&)=default;//Fails
#else
C(const C&c):B(c){};//Works
#endif
};
C<A> d;
int main(){}

produces the following error

$g++ -std=c++11 main.cpp
main.cpp: In instantiation of 'struct C<A>':
main.cpp:9:6:   required from here
main.cpp:4:1: error: 'C<B>::C(const C<B>&) [with B = A]' declared to take const
reference, but implicit declaration would take non-const
 C(const C&)=default;//Fails
 ^

Looks like defaulted function is implicitly instantiated even if it's not used
(it's not so for user-defined functions).

GCC 4.4 worked well with this case.