$ cat u.f90 
  if (isscan () /= 0) call abort
contains
  integer function isscan (substr)
    character(*), optional :: substr
    if (.not.present(substr)) isscan = myscan ("foo", "over")
  end function isscan
end
$ gfortran u.f90 
u.f90: In function ‘MAIN__’:
u.f90:5: internal compiler error: Segmentation fault


-- 
           Summary: ICE with character optional arg
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: ice-on-valid-code
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: fxcoudert at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29322