[Bug c++/96445] New: extern template results in missing constructor symbol

[Bug c++/96445] New: extern template results in missing constructor symbol

[tyu at eridex dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96445

            Bug ID: 96445
           Summary: extern template results in missing constructor symbol
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tyu at eridex dot org
  Target Milestone: ---

Created attachment 48989
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48989&action=edit
g++ -v output

The following results in: undefined reference to `C<int>::C(int)'

template <typename T>
class C {
  private:
    constexpr C(T){};
  public:
    constexpr static C<T> make(T t) { return C<T>(t); }
};
extern template class C<int>;
inline constexpr C<int> constant = C<int>::make(0);
template class C<int>;
int main() { return 0; }

Compiled with:
$ g++ -v -save-temps -std=c++2a -Wall -Wextra issue.cpp > output.txt 2>&1

output.txt is attached.

[Bug c++/96445] extern template results in missing constructor symbol

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96445

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |link-failure
                 CC|                            |jason at gcc dot gnu.org

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
extern template class C<int>;
template class C<int>;

combo probably does not instanitate C<int>.

[Bug c++/96445] extern template results in missing constructor symbol

[tyu at eridex dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96445

--- Comment #2 from tyu at eridex dot org ---
The extern template and constant are what would appear in the header file for
class C. The explicit instantiation would appear in the source file:

// -- C.h ------------
template <typename T>
class C {
  private:
    constexpr C(T){};
  public:
    constexpr static C<T> make(T t) { return C<T>(t); }
};
extern template class C<int>;
inline constexpr C<int> constant = C<int>::make(0);

// -- C.cpp ----------
template class C<int>;

// -- main.cpp -------
int main() { return 0; }
// -------------------