Is this a valid standard-conforming Fortran code:
module NonOptionalArg_mod contains subroutine nonOptionalArg_func(optionalArg) integer, intent(in) :: optionalArg !write(*,*) "optionalArg: ", optionalArg end subroutine nonOptionalArg_func end module NonOptionalArg_mod module OptionalArg_mod use NonOptionalArg_mod contains subroutine optionalArg_func(optionalArg) integer, intent(in), optional :: optionalArg call nonOptionalArg_func(optionalArg) end subroutine optionalArg_func end module OptionalArg_mod program optionalProg use OptionalArgFirst_mod implicit none call optionalArg_func() end program optionalProg
The main program calls a subroutine that takes an optional argument. The function then passes the non-existing optional argument to another function that takes a non-optional argument. The code compiles fine with ifort 19.4 and as long the optional argument is not used, it does not lead to any runtime errors.
By passing an optional argument as an actual argument to a subprogram, which in turn expects that argument to be always present, you deprive yourself of the ability to check in that subprogram whether the argument is present and only use it if it is present.
Another way of viewing the situation is to regard an optional argument as being represented by a null-pointer. You can pass that null-pointer around, as long as it never gets dereferenced or checked for nullity. This is somewhat analogous to having an argument that is allocatable in the caller but has not been declared as allocatable in the callee.
In your program, on Line-19, "use OptionalArgFirst_mod" should have been "use OptionalArg_mod"
The standard says "a nonoptional dummy argument shall be present". In your example, it isn't, so your program is non-conforming. The standard does not require detection of this behavior and you should not rely on what happens now.