[Bug fortran/78659] New: Spurious "requires DTIO" reported against namelist statement

[ian_harvey at bigpond dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78659

            Bug ID: 78659
           Summary: Spurious "requires DTIO" reported against namelist
                    statement
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ian_harvey at bigpond dot com
  Target Milestone: ---

The attached, when compiled with gfortran recent trunk (r243203): 


MODULE ma
  IMPLICIT NONE
  TYPE :: ta
    INTEGER, ALLOCATABLE :: array(:)
  END TYPE ta
END MODULE ma

PROGRAM p
  USE ma
  TYPE(ta) :: x
  NAMELIST /nml/ x
END PROGRAM p


results in:


$ gfortran 2016-12-03\ dtio-namelist-1.f90
2016-12-03 dtio-namelist-1.f90:11:15:

   NAMELIST /nml/ x
               1
Error: NAMELIST object ‘x’ in namelist ‘nml’ at (1) has ALLOCATABLE or POINTER
components and thus requires a defined input/output procedure


The error is reported against the namelist statement.  The standard does not
impose such a requirement on the namelist statement itself - the requirement is
imposed on data transfer statements that reference the namelist (F2008
9.6.4.7p2).

A namelist with such an object is not useful, so a warning is perhaps warranted
specifically for the above code, but the current error message causes issues
with useful code, such as:


MODULE mb
  IMPLICIT NONE
  TYPE :: tb
    INTEGER, ALLOCATABLE :: array(:)
  CONTAINS
    PROCEDURE :: read_formatted
    GENERIC :: READ(FORMATTED) => read_formatted
  END TYPE tb
CONTAINS
  SUBROUTINE read_formatted(dtv, unit, iotype, v_list, iostat, iomsg)
    CLASS(tb), INTENT(INOUT) :: dtv
    INTEGER, INTENT(IN) :: unit
    CHARACTER(*), INTENT(IN) :: iotype
    INTEGER, INTENT(IN) :: v_list(:)
    INTEGER, INTENT(OUT) :: iostat
    CHARACTER(*), INTENT(INOUT) :: iomsg

    iostat = 0
  END SUBROUTINE read_formatted
END MODULE mb

PROGRAM p
  USE mb
  TYPE(tb) :: y
  NAMELIST /nml/ y
  READ (*, nml)
END PROGRAM p


$ gfortran 2016-12-03\ dtio-namelist-2.f90
2016-12-03 dtio-namelist-2.f90:25:15:

   NAMELIST /nml/ y
               1
Error: NAMELIST object ‘y’ in namelist ‘nml’ at (1) has ALLOCATABLE or POINTER
components and thus requires a defined input/output procedure


The error goes away if a defined output procedure is also provided, but the
standard does not require a defined output procedure for the last example.