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A general description is here.
The interpretation of source code with extension ‘.f’ is changing. Previously this denoted FORTRAN 77 code, but current compilers no longer have a FORTRAN 77 mode and interpret it as ‘fixed-form’ Fortran 90 (or later where supported) code. Extensions ‘.f90’ and ‘.f95’ continue to indicate ‘free-form’ Fortran code.
Legal FORTRAN 77 code is also legal fixed-form Fortran 9x; however this change legitimizes the use of later features, in particular to replace features marked ‘obsolescent’ in Fortran 90 and ‘deleted’ in Fortran 2018 which ‘gfortran’ 8.x and later warn about.
‘R_unif_index’ is now considered to be part of the C API.