[Bug c++/57728] New: Explicit template instantiation with defaulted method causes missing symbol

[Bug c++/57728] New: Explicit template instantiation with defaulted method causes missing symbol

[bmerry at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57728

            Bug ID: 57728
           Summary: Explicit template instantiation with defaulted method
                    causes missing symbol
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bmerry at gmail dot com

Created attachment 30378
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30378&action=edit
Minimal test case

I don't claim to fully understand all the intricies of C++11, but the following
smells fishy to me. I am using a combination of features:
1. The = default syntax to restore implicit constructors/assignments that were
otherwise hidden (e.g. default constructor when there are no user-defined
constructors), in a templated class.
2. "extern template" in the header to suppress instantiation of a specific
instance, with an explicit instantiation in one translation unit.

In some cases (I assume depending on compiler eliding) this causes a link-time
error for the defaulted constructor. I have attached a minimal test case. I
don't know whether there is supposed to be a symbol or whether the compiler is
supposed to inline the default implementation, but currently there is a
mismatch.

System information: Ubuntu 12.04 with
gcc version 4.8.1 (Ubuntu 4.8.1-2ubuntu1~12.04) 

Output (there's a more detailed log in the attachment with -v -save-temps):
g++-4.8 -std=c++11 -c defaulted.cpp
g++-4.8 -std=c++11 -c impl.cpp
g++-4.8 -std=c++11 -o defaulted defaulted.o impl.o
defaulted.o: In function `main':
defaulted.cpp:(.text+0x10): undefined reference to `A<int>::A()'
collect2: error: ld returned 1 exit status
make: *** [defaulted] Error 1

[Bug c++/57728] Explicit template instantiation with defaulted method causes missing symbol

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57728

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-06-26
                 CC|                            |jason at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The explicit instantiation declaration suppresses the definition of A<int>::A()
in defaulted.o, but the explicit instantiation definition doesn't cause that
symbol to be emitted in impl.o, so when that constructor is not inlined there
is no definition.

As a single file:

template<typename T>
struct A
{
    T x;
    A() = default;
    A(const A &other) = delete;
};

extern template class A<int>;

int main()
{
    A<int> a;
}

This compiles with clang but not G++ because Clang doesn't create a reference
to A<int>::A() from main(), so it doesn't matter that the explicit
instantiation is not defined in the program.

[Bug c++/57728] Explicit template instantiation with defaulted method causes missing symbol

[bmerry at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57728

--- Comment #2 from Bruce Merry <bmerry at gmail dot com> ---
(In reply to Jonathan Wakely from comment #1)
> The explicit instantiation declaration suppresses the definition of
> A<int>::A() in defaulted.o, but the explicit instantiation definition
> doesn't cause that symbol to be emitted in impl.o, so when that constructor
> is not inlined there is no definition.

That's more or less what I figured was happening. Can you clarify whether you
think this a GCC bug or just me misunderstanding the language? Thanks.

[Bug c++/57728] Explicit template instantiation with defaulted method causes missing symbol

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57728

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's a bug

[Bug c++/57728] Explicit template instantiation with defaulted method causes missing symbol

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57728

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |edward.hades at gmail dot com

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
*** Bug 58078 has been marked as a duplicate of this bug. ***