missing-default-case (C011)

This rule is turned on by default.

What it does

Checks that select case statements have a case default.

Why is this bad?

Select statements without a default case can lead to incomplete handling of the possible options. If a value isn't handled by any of the cases, the program will continue execution, which may lead to surprising results. This is a common source of bugs when adding new options, as it's easy to forget to update all select statements. Unfortunately, because Fortran doesn't have proper enums, it's not possible for the compiler to issue warnings for non-exhaustive cases. Having a default case allows for the program to gracefully handle errors.

Examples

Instead of:

select case(ntype)
case (1)
    call routine1()
case (2)
    call routine2()
end select

use:

select case(ntype)
case (1)
    call routine1()
case (2)
    call routine2()
case default
    call handle_error("Invalid ntype: ", ntype)
end select

If you do only intend to handle a subset of cases, you can use a continue statement with an explanatory comment:

select case(ntype)
case (1)
    call routine1()
case (2)
    call routine2()
case default
    ! Other ntypes handled elsewhere
    continue
end select

You may also consider instead using an if statement. This can make your intention more obvious.