Thanks very much for the additional info! However…, as far as I understand from the linked thread above, -warn declarations does not correspond to -fimplicit-none, and so the OP (in that thread) wishes the latter? Or possibly, is there a way to selectively turn -warn declarations only to an error (without affecting other warning options)?
I think it would be useful if an exact counterpart to -fimplicit-none is available in all the major compilers (particularly Intel Fortran), but if not, the “use -u” argument linked above (https://j3-fortran.org/doc/year/22/22-182.txt) does not seem to make sense (to me).