Strange behaviour when using type(*) and character(*) in GFortran

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Note that the argument to sub must be allocatable or optional then in your example. As written the call to is_unallocated should always return .true., as it is always present, but you may get a segfault if the actual argument to sub isn’t allocated. I.e. it should be

    subroutine sub(useless_var, print_this)
        type(*), intent(in), optional      :: useless_var(..)
        character(*), intent(in) :: print_this
        print *, "should be 123: '", print_this, "'"
        print *, "useless_var is unallocated", is_unallocated(useless_var)
    end subroutine sub

Note that type(*) is fine in this case if you don’t intend to use its value.