Why do c++ standard library template class functions say undefined reference while linking if they are defined in a header

When using <fstream> with g++ -D_GLIBCXX_HAVE__WFOPEN=1 -D_GLIBCXX_USE_WCHAR_T=1(msys2 gcc 13.3.0) it throws an error

/usr/lib/gcc/x86_64-pc-msys/13.3.0/../../../../x86_64-pc-msys/bin/ld: /tmp/cceEWaVo.o:x.cpp:(.text+0x50): undefined reference to `std::basic_ifstream<char, std::char_traits<char> >::open(wchar_t const*, std::_Ios_Openmode)'

code:

#include <fstream>int main() {        std::ifstream f("x.cpp"); // works        f.close();        f.open(L"x.cpp"); // doesn't link        return 0;}

even thou they are defined in <bits/fstream.tcc> witch is included at the end of <fstream>

I was expecting the code to compile normally since ifstream is a template class and is defined in a header.I checked if the code in <bits/fstream.tcc> that defines the open(wchar_t const* __s, std::ios_base::openmode __mode) is detected by the compiler by editing the header file and putting a #error block right above the function and the compiler found if (which means that all the ifdefs where passed)

Could someone please explain why header-defined template class functions need to also somehow exist in libstdc++

I am aware that the mingw-w64 gcc compiler works with fstream open(wchar_t const* __s, std::ios_base::openmode __mode) but I specificly need the msys version