GCC Gfortran and Intel oneAPI are the most advanced modern Fortran compilers. Useful Fortran 2018 enhancements include: select rank assumed array rank, error stop within pure procedures, random_init to initialize random number seed, and implicit none (type, external) to require external procedures to be explicitly declared. GCC 8 is the oldest version currently maintained. Intel oneAPI also has full Fortran 2018 support.

To get recent GCC is usually straightforward. Red Hat should use devtoolset. MacOS Homebrew quickly adds the lastest GCC version. If Ubuntu gfortran repo defaults aren’t adequate, get recent Gfortran via PPA.

Here are some of the major changes in Gfortran by version:

• Gfortran 10 added select rank
• Gfortran 9 added random_init() to initialize the random generator seed…randomly
• Gfortran 8.1 added automatic nested loop exchange with do concurrent, actual argument array with too few elements for dummy argument now errors, initial support for parameterized derived types (simply define kind at initialization) and coarray support for teams. Gfortran 8.2 and 8.3 did not introduce new Fortran features.
• Gfortran 8.0 added -std=f2018 and deprecated -std=f2008ts.
• Gfortran 7 added derived type IO select type et all–complete Fortran 2003, Fortran 2018 non-constant stop and error stop codes, and -fdec- options to help compile very old non-standard code.
• Gfortran 6 added Fortran 2008 submodule support, useful for large projects to save compilation time and allow powerful use scenarios. Fortran 2003 deferred-length character are useful for avoiding bothersome trim() everywhere.
• GCC 5 added full support for OpenMP 4.0, Fortran 2003 ieee_ intrinsics, Fortran 2008 error stop in pure procedures with constant error code.

GCC 4.9 added Fortran 2003 deferred-length character variables in derived types. GCC 4.8 supported Fortran 2008 polymorphism, including select type, class(*), type(*), and assumed rank dimension(..). GCC 4.6 was the first version of Gfortran reaching beyond Fortran 95, with Fortran 2003 deferred-length character variable and Fortran 2008 impure elemental support. GCC 4.5 added Fortran 2008 iso_fortran_env. GCC 4.4 added initial support for polymorphism and OpenMP 3.

CMake allows switching parameters based on compiler version. This is very useful for modern Fortran programs.

Example CMakeLists.txt for Fortran compiler version dependent options.

if(CMAKE_Fortran_COMPILER_ID STREQUAL GNU)

  # option applying to any language for this compiler
  add_compile_options(-mtune=native)

  # language-specific, note LEADING space
  string(APPEND CMAKE_Fortran_FLAGS " -fimplicit-none")

  if(CMAKE_Fortran_COMPILER_VERSION VERSION_GREATER_EQUAL 8)
    string(APPEND CMAKE_Fortran_FLAGS " -std=f2018")
  endif()

endif()

add_executable(myprog main.f90)

Reference: Gfortran changelog