tempfile is a Debian specific utility for creating temporary files. Its man page notes:
tempfile is deprecated; you should use mktemp(1) instead.
mktemp are POSIX, but
tempfile is Debian specific while
mktemp works on GNU, OSX, BusyBox, *BSD and Solaris.
ShellCheck will not recognize when a function overrides this name.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.