a) Accepts invalid with coindexed variables. Fortran 2008:

R804  association  is  associate-name => selector
R805  selector  is  expr
                or  variable
C801 (R804) If selector is not a variable or is a variable that has a vector subscript, associate-name shall not appear in a variable definition context (16.6.7).
C802 (R804) An associate-name shall not be the same as another associate-name in the same associate-stmt.
C803 (R805) variable shall not be a coindexed object.
C804 (R805) expr shall not be a variable.


I think that prohibits:
  associate (myname => coarray[i])

For ASSOCIATE, one gets:

associate (x => caf[4])
                      1
Error: Association target at (1) must not be coindexed


But for SELECT TYPE, it is accepted:

module gn
  type :: ncb
  end type ncb
  type, public :: tn
     class(ncb), allocatable :: cb(:)[:]
  end type tn
contains
  integer function name(self)
    implicit none
    class (tn), intent(in) :: self
    select type (component => self%cb(1)[4])  ! INVALID due to the "[4]"
    end select
  end function name
end module gn


Note: The problem is not the coarray, just the coindex (gfc_is_coindexed).



b) ICE with the following:
    select type (t = self%cb)
Note the "=" opposed to "=>". Backtrace:

0x5e32d0 gfc_undo_symbols()
        ../../gcc/fortran/symbol.c:2972
0x5af3aa reject_statement
        ../../gcc/fortran/parse.c:1747
0x5af4dc match_word
        ../../gcc/fortran/parse.c:72


module gn
  type :: ncb
  end type ncb
  type, public :: tn
     class(ncb), allocatable :: cb
  end type tn
contains
  integer function name(self)
    implicit none
    class (tn), intent(in) :: self
    select type (t = self%cb)
    end select
  end function name
end module gn



c) The following also ICEs:

0x5cd395 resolve_select_type
        ../../gcc/fortran/resolve.c:7791
0x5cd395 resolve_code
        ../../gcc/fortran/resolve.c:9707


module gn
  type :: ncb
  end type ncb
  type, public :: tn
     type(ncb), allocatable :: cb
  end type tn
contains
  integer function name(self)
    implicit none
    class (tn), intent(in) :: self
    select type (t => self%cb)
    type is (ncb)
        t = 5
    end select
  end function name
end module gn