[Bug fortran/86408] New: [9 Regression] bogus error: ABSTRACT INTERFACE must not have an assumed character length result (F2003: C418)

[janus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86408

            Bug ID: 86408
           Summary: [9 Regression] bogus error: ABSTRACT INTERFACE must
                    not have an assumed character length result (F2003:
                    C418)
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janus at gcc dot gnu.org
  Target Milestone: ---

Test case:


module m

   implicit none

   type, abstract :: t
   contains
      procedure(ifc), deferred :: tbf
      procedure :: tbs
   end type

   abstract interface
      function ifc(x) result(str)
         import :: t
         class(t) :: x
         character(len=:), allocatable :: str
      end function
   end interface

contains

   subroutine tbs(x)
      class(t) :: x
      print *, x%tbf()
   end subroutine

end


This example (which I'd say is perfectly valid code), compiles well with
gfortran 8 (and ifort/pgfortran/flang), but is rejected by recent trunk builds
with:

       function ifc(x) result(str)
      1
Error: ABSTRACT INTERFACE ‘ifc’ at (1) must not have an assumed character
length result (F2003: C418)


This error is apparently bogus, primarily because 'str' is not assumed-length,
but deferred-length.

Moreover the error message does not really match the quoted standard clause
F03:C418, which says:

"A function name declared with an asterisk type-param-value shall not be an
array, a pointer, recursive, or pure."

1) This does not talk about abstract interfaces at all, but functions in
general.
2) The function in the examples has no asterisk type-param-value.
3) The function in the example is neither an array, pointer, recursive or pure.