[Bug fortran/78443] New: Incorrect behavior with non_overridable keyword

[perini at wisc dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78443

            Bug ID: 78443
           Summary: Incorrect behavior with non_overridable keyword
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: perini at wisc dot edu
  Target Milestone: ---

Created attachment 40094
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40094&action=edit
Minimal program to test the non_overridable keyword issue

Hi, 

I've been having some issues recently when using the non_overridable keyword on
an extended derived type, with gfortran 5.3.0 and 6.1.0. 

In essence, when the non_overridable function is called from: 
- A type which extends an abstract type -> segmentation fault (looks like call
to null function pointer);
- A type which extends a non-abstract type -> call the parent's function
instead than the own function.

"Modern Fortran" by Metcalf Reid Cohen states: 
- If the non_overridable attribute appears, that type-bound procedure cannot be
overridden during type extension; 
- non_overridable is incompatible with deferred, since that requires the
type-bound procedure to be overridden; 
- Overriding a type-bound procedure is not permitted if the inherited one has
the non_overridable attribute

So I'm guessing that using the non_overridable keyword in a non-abstract
extended type should be OK and prevent the function being overridden in further
type extensions. A bit more discussion has taken place in
http://stackoverflow.com/questions/40510423/segmentation-fault-with-deferred-functions-and-non-overridable-keyword

The attached code reproduces the error.

Thanks for the great work on gfortran!

Federico