There is a bug introduced in recent versions of gfortran (6.4.1, 7.3.1 ). These versions raise an internal compiler error when compiling FoX.

I made a bug report to gcc ( https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85526 ).

As far as I understand the bug is related to the usage of the len interface to compute the dimensions of the results of character type of many pure functions used for example in fox_m_fsys_format.F90 or in
m_dom_dom.F90.

A workaround that seems to work is to move the routines involved in the len interface at the bottom of their modules, but as explained by a note in the code this clashes with an explicit fortran specification and the modified source is not accepted as valid by other compilers.

A more stable workaround could be to move the len interface to a new module and import the interface.