[Bug c++/119527] New: [modules] Internal linkage virtual key function of template in header unit is not emitted

[nshead at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119527

            Bug ID: 119527
           Summary: [modules] Internal linkage virtual key function of
                    template in header unit is not emitted
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nshead at gcc dot gnu.org
            Blocks: 103524
  Target Milestone: ---

Consider:

  // header.hpp
  namespace {
    template <typename T> struct S {
      virtual void key() {}
    };
    S<int> s;
  }

  // main.cpp
  import "header.hpp";
  int main() {}

$ g++-15 -fmodule-header header.hpp
header.hpp:3:18: warning: ‘void {anonymous}::S<T>::key() [with
<template-parameter-1-1> = int]’ used but never defined
    3 |     virtual void key() {}
      |                  ^~~
$ g++-15 -fmodules -fno-module-lazy main.cpp
/usr/bin/ld: /tmp/cc5etnIe.o:(.rodata+0x10): undefined reference to `(anonymous
namespace)::S<int>::key()'
collect2: error: ld returned 1 exit status

The issue seems to be related to header modules skipping much of
'c_parse_final_cleanups', since by not emitting vtables there they sometimes
miss instantiating template bodies, which leads to not only a confusing-looking
warning when building the header but also results in the body never being
emitted confusing importers as well.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103524
[Bug 103524] [meta-bug] modules issue