[Bug fortran/78662] New: Incorrect parsing of quotes in the char-literal-constant of the DT data descriptor

["ian_harvey at bigpond dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78662

            Bug ID: 78662
           Summary: Incorrect parsing of quotes in the
                    char-literal-constant of the DT data descriptor
           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 following, when compiled with gfortran recent trunk, generates an error
that indicates that parsing of the optional character literal constant of a DT
edit descriptor is not correctly handling embedded quotes.

MODULE m
  IMPLICIT NONE

  TYPE :: t
    CHARACTER :: c
  CONTAINS
    PROCEDURE :: write_formatted
    GENERIC :: WRITE(FORMATTED) => write_formatted
  END TYPE t
CONTAINS
  SUBROUTINE write_formatted(dtv, unit, iotype, v_list, iostat, iomsg)
    CLASS(t), INTENT(IN) :: dtv
    INTEGER, INTENT(IN) :: unit
    CHARACTER(*), INTENT(IN) :: iotype
    INTEGER, INTENT(IN) :: v_list(:)
    INTEGER, INTENT(OUT) :: iostat
    CHARACTER(*), INTENT(INOUT) :: iomsg

    WRITE (unit, "(A)", IOSTAT=iostat, IOMSG=iomsg) iotype
  END SUBROUTINE write_formatted
END MODULE m

PROGRAM p
  USE m
  IMPLICIT NONE

  TYPE(t) :: x
  WRITE (*, "(DT'a''b')") x
END PROGRAM p


$ gfortran 2016-12-03\ dt.f90  && ./a.out
2016-12-03 dt.f90:28:21:

   WRITE (*, "(DT'a''b')") x
                     1
Error: Unexpected element ‘'’ in format string at (1)


The delimiter of the inner character literal constant (embedded in the
character literal constant that makes up the entire format specification) is a
single quote - the two single quotes in sequence therefore represent a single
single quote in the value that the inner character literal represents.

A more complicated variant is:

  WRITE (*, '(DT''a''''b'')') x

The problem also arises with FORMAT statements, which don't have the complexity
associated with having to consider "nested" literals.

  WRITE (*, 100) x
  100 FORMAT(DT'a''b')