Found the following significant warnings:
  fdiff_fgrowth.cpp:606:1: warning: control reaches end of non-void function [-Wreturn-type]
  fdiff_fgrowth.cpp:1209:1: warning: control reaches end of non-void function [-Wreturn-type]
  fdiff_fgrowth.cpp:1799:1: warning: control reaches end of non-void function [-Wreturn-type]
  flag.cpp:216:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:222:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:228:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:234:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:470:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:476:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:482:1: warning: no return statement in function returning non-void [-Wreturn-type]
  flag.cpp:488:1: warning: no return statement in function returning non-void [-Wreturn-type]
  mrtl_mctl.cpp:49:1: warning: no return statement in function returning non-void [-Wreturn-type]
  mrtl_mctl.cpp:54:1: warning: no return statement in function returning non-void [-Wreturn-type]
  mrtl_mctl.cpp:109:1: warning: no return statement in function returning non-void [-Wreturn-type]
  mrtl_mctl.cpp:114:1: warning: no return statement in function returning non-void [-Wreturn-type]
  psmat.cpp:54:1: warning: no return statement in function returning non-void [-Wreturn-type]
  psmat.cpp:59:1: warning: no return statement in function returning non-void [-Wreturn-type]
  psmat.cpp:64:1: warning: no return statement in function returning non-void [-Wreturn-type]
  pwnobs.cpp:28:1: warning: no return statement in function returning non-void [-Wreturn-type]
  pwnobs.cpp:33:1: warning: no return statement in function returning non-void [-Wreturn-type]
  pwnobs.cpp:38:1: warning: no return statement in function returning non-void [-Wreturn-type]
  pwnobs.cpp:43:1: warning: no return statement in function returning non-void [-Wreturn-type]
  seqid_groupid.cpp:300:1: warning: no return statement in function returning non-void [-Wreturn-type]
  seqid_groupid.cpp:305:1: warning: no return statement in function returning non-void [-Wreturn-type]
  seqid_groupid.cpp:310:1: warning: no return statement in function returning non-void [-Wreturn-type]
  seqid_groupid.cpp:315:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:59:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:64:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:69:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:74:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:101:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:106:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:111:1: warning: no return statement in function returning non-void [-Wreturn-type]
  varying.cpp:116:1: warning: no return statement in function returning non-void [-Wreturn-type]
See ‘/home/hornik/tmp/R.check/r-devel-gcc/Work/PKGS/collapse.Rcheck/00install.out’ for details.
* used C compiler: ‘gcc-12 (Debian 12.2.0-10) 12.2.0’
* used C++ compiler: ‘g++-12 (Debian 12.2.0-10) 12.2.0’