I definitely must use another feature to check as -fcoarray=single is supported since GCC 4.6 while there are several Fortran features supported since GCC 9.0 :(
16.06.2022, 01:52, "Torokhov Sergey" <torokhov-s-a@××××××.ru>:
Currently the fortran-2.eclass Fortran compiler dialect is restricted with check of Fortran 2003 standard.
Some packages could require to check at least Fortran 2008 support.
The proposed attached patch offer to check Fortran 2008 compatibility via one of the new most notable feature - Coarrays support (parallel processing feature).
Besides the new test file the current check require the passing of GCC (gfortran) compiler option `-fcoarray=single` to check Fortran 2008 compatibility.
The same patch is available via GIthub pull request: https://github.com/gentoo/gentoo/pull/25920