[Bug c++/64293] New: [C++14] explicit instantiation declaration suppresses instantiation of constexpr constructor

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64293

            Bug ID: 64293
           Summary: [C++14] explicit instantiation declaration suppresses
                    instantiation of constexpr constructor
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

template <typename T>
struct Foo {
  constexpr Foo()  { }

  constexpr Foo(T const& val) {
    for (auto& el : _data) {
      el = val;
    }
  }

  T _data[2];
};

extern template struct Foo<int>;

#ifdef USE
Foo<int> f = 1;
#else
template struct Foo<int>;
#endif


When compiled with -std=gnu++14 this does not instantiate the constructor:

$ g++14 -c foo.cpp && nm -C foo.o
                 U Foo<int>::Foo(int const&)
0000000000000000 W Foo<int>::Foo()
0000000000000000 W Foo<int>::Foo()

And when compiled so it uses the constructor, the explicit instantiation
declaration suppresses the instantiation of that constructor:

$ g++14 -c foo.cpp -DUSE && nm -C foo.o
0000000000000038 t _GLOBAL__sub_I_f
0000000000000000 t __static_initialization_and_destruction_0(int, int)
                 U Foo<int>::Foo(int const&)
0000000000000000 B f

So neither the file that uses it nor the file that should define it has the
symbol, leading to a linker error.

The symbol is defined if:
- the constructors are not constexpr, or
- the constructor is not delegated to by the default constructor, or
- the constructor does not contain a loop, or
- the explicit instantation is not declared before being defined

Oddly, clang 3.4 has almost exactly the same bug, the only difference is that
the presence of the loop makes no difference, the constructor is not generated
even if the body is empty. Clang 3.5 generates the constructor as I expect.