In some module where .cxx files are compiled and a shared library is built from
them, do the following:

1  Create a fresh (internal) header foo.hxx that contains a fresh function
declaration "void foo();" (but no definition).
2  In one of the .cxx files, add "#include "foo.hxx" and a call to foo().
3  Build the module, which will fail when linking the shared library, due to the
missing definition of foo().  (And because of the linking failure, the
dependency from the .cxx file to foo.hxx is not updated, because that step would
come afterwards, as Ause says.)
4  Fix foo.hxx to also include an (inline) definition of foo, by changing it to
"void foo {}".
5  Build again:  Linking will fail again as the .cxx will not be recompiled due
to the missing dependency.