https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119751
--- Comment #7 from Zbigniew <zbigniew2011 at gmail dot com> --- I'd like to stress it: presently these error messages (I mean without '-fiso' option) look like compiler was malfunctioning: coex.mod:8:24: error: In program module 'CoEx': unknown symbol 'COROUTINE' 8 | FROM COROUTINES IMPORT COROUTINE, NEWCOROUTINE, TRANSFER; | ^~~~~~~~~ coex.mod:8:35: error: unknown symbol 'NEWCOROUTINE' 8 | FROM COROUTINES IMPORT COROUTINE, NEWCOROUTINE, TRANSFER; | ^~~~~~~~~~~~ coex.mod:8:49: error: unknown symbol 'TRANSFER' 8 | FROM COROUTINES IMPORT COROUTINE, NEWCOROUTINE, TRANSFER; | ^~~~~~~~ coex.mod:44:9: error: the following unknown symbols in module 'CoEx' were locally used: NEWCOROUTINE, COROUTINE and TRANSFER 44 | END CoEx. | ^ As one can find out: the module exists, these symbols can be found inside, so the only guess — for anyone who's not (yet) expert in Modula 2 — is „must be something wrong with the libraries”. And after I found similar module in sibling sub-dir my conclusion was: „conflicting libraries, probably” (which has been „confirmed” when I renamed the file, and when I found even more difficult issue in 12.2).