implicit-external-procedures (C003)

This rule is turned on by default.

What it does

Checks if implicit none is missing external

Why is this bad?

implicit none disables implicit types of variables but still allows implicit interfaces for procedures. Fortran 2018 added the ability to also forbid implicit interfaces through implicit none (external), enabling the compiler to check the number and type of arguments and return values.

implicit none is equivalent to implicit none (type), so the full statement should be implicit none (type, external).

This rule is only active when targeting Fortran 2018 or later.