Gfortran and .smod files

[ashe]

@everythingfunctional

It is valid (albeit usually useless) to define a submodule even if there is nothing in the module implying that one would need to exist.

I don’t think that’s true. C1412 (of Fortran 2018) says:

The ancestor-module-name shall be the name of a nonintrinsic module that declares a separate module procedure

And with your example, I get this error from gfortran 12.2.0:

f951: Fatal Error: Module file ‘mod.smod’ has not been generated, either because the module does not contain a MODULE PROCEDURE or there is an error in the module.

This might help explain why gfortran generates .smod files even when there are no submodules. There seems to be a .smod generated for modules that can have submodules so gfortran can perform that check.