[Bug c++/109464] New: gcc does not instantiate constructor for explicitly instantiated template

["lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109464

            Bug ID: 109464
           Summary: gcc does not instantiate constructor for explicitly
                    instantiated template
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

Created attachment 54824
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54824&action=edit
unreduced testcase

Attached is a preprocessed file. I don't know how to reduce it for absence of a
function.


```
template<typename charT>
class basic_shallow_string
  {
    static_assert(!is_array<charT>::value, "invalid character type");
    static_assert(is_trivial<charT>::value, "characters must be trivial");
    template<typename, typename> friend class basic_cow_string;

  private:
    const charT* m_ptr;
    size_t m_len;

  public:
    explicit constexpr
    basic_shallow_string(const charT* ptr) noexcept
      : m_ptr(ptr), m_len(noadl::xstrlen(ptr))
      { }

  public:
    constexpr
    const charT*
    c_str() const noexcept
      { return this->m_ptr;  }

    constexpr
    size_t
    length() const noexcept
      { return this->m_len;  }

(... more member functions follow ...)
  };

extern template class basic_shallow_string<char>;
```

And in 'cow_string.cpp' I have 

```
#include "cow_string.hpp"
namespace rocket {

template class basic_shallow_string<char>;

(... more explicit instantiations follow ...)
}  // namespace rocket
```

This explicit instantiation instantiates all inline member functions above, but
not the constructor. I have examined assembly output, and there is no
constructor. Unless optimization is enabled, this causes undefined references:

```
/usr/bin/ld: asteria/repl/bin_asteria-commands.o: in function
`rocket::basic_shallow_string<char> rocket::sref<char>(char const*)':
/home/lh_mouse/GitHub/asteria/asteria/repl/../../rocket/cow_string.hpp:97:
undefined reference to
`rocket::basic_shallow_string<char>::basic_shallow_string(char const*)'
collect2: error: ld returned 1 exit status
```

This happens only with the constructor.