[Bug c++/96853] New: Explicit template instantiation & thread_local interaction

["tobias.bruell at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96853

            Bug ID: 96853
           Summary: Explicit template instantiation & thread_local
                    interaction
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tobias.bruell at gmail dot com
  Target Milestone: ---

Compiling the below with

> g++-10 main.cpp library.cpp -o main

in gcc 10.1 leads to an executable "main" that segfaults. The problem seems to
be that the out-commented "extern" declarations in library.hpp are missing.

So, the program might be UB, but it would be nice to get an error message from
the compiler (as Clang does).

Otherwise, the linker could give an error: the final executable "main" contains
a "callq 0x0" instruction (on x86_64) if you also add "-static" to the above
command-line. I think the problem is that "_ZTHN7library3FooIiE1fE" (TLS init
function for library::Foo<int>::f) is missing from any of the intermediate
object files.

--------------------------------
--------- library.hpp ----------

#ifndef INCLUDE_GUARD_LIBRARY_H_
#define INCLUDE_GUARD_LIBRARY_H_

namespace library
{
  template<typename T>
  struct Foo
  {
    using Func = T (*) (T);
    static thread_local Func f;
  };

  //extern template struct Foo<char>;
  //extern template struct Foo<int>;
  //extern template struct Foo<long>;
}

#endif

-------------------------------
--------- library.cpp ---------

#include "library.hpp"

namespace library
{
  template<typename T>
  T core_function (T val)
  {
    return val * 2;
  }

  template<typename T>
  thread_local
  typename Foo<T>::Func
  Foo<T>::f = &core_function<T>;

  template struct Foo<char>;
  template struct Foo<int>;
  template struct Foo<long>;
}

--------------------------------
----------- main.cpp -----------

#include "library.hpp"

int main () {
  return library::Foo<int>::f(2);
}