Fortran compilers typically have options for enforcing Fortran standards. The compilers raise additional warnings or errors for code that is not deemed compliant. Fortran standard options can make false warnings, so we generally do not enable standards checking for user defaults. However, we do enforce implicit none as a quality measure.

It’s also important to use implicit none so that each variable must be assigned beforehand. We recommend the Fortran 2018 statement:

implicit none (type, external)

which requires explicitly defined procedure interfaces as well.

type

the traditional implicit none default

external

new for Fortran 2018, requires explicit interface for external procedures.

GCC Gfortran -std=f2018 enforces Fortran 2018 standard. Consider these Gfortran options:

gfortran -Wall -fimplicit-none

Intel oneAPI -stand f18 enforces Fortran 2018 standard. Consider these options that also enforce implicit none:

ifx -warn

Cray Fortran compiler enforces implicit none via option:

ftn -eI

note that’s a capital “I” not a lowercase “ell”.

Nvidia HPC Fortran compiler enforces implicit none via:

nvfortran -Mdclchk

NAG Fortran has -f2018 Fortran 2018 flag. Enforce implicit none by:

nagfor -u

CMake logic to enforce these standards:

if(CMAKE_Fortran_COMPILER_ID STREQUAL "Cray")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-eI>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
  add_compile_options(-Wall "$<$<COMPILE_LANGUAGE:Fortran>:-fimplicit-none>")
elseif(CMAKE_Fortran_COMPILER_ID MATCHES "^Intel")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-warn>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "NVHPC")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-Mdclchk>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "NAG")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-u>")
endif()