[Bug fortran/78640] New: [F2015] gfortran accepts invalid allocatable polymorphic result in pure function

[damian at sourceryinstitute dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78640

            Bug ID: 78640
           Summary: [F2015] gfortran accepts invalid allocatable
                    polymorphic result in pure function
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: damian at sourceryinstitute dot org
  Target Milestone: ---

Constraint C1589 in Draft 16-007r2 of the Fortran 2015 standard states

"A pure function shall not have a polymorphic allocatable result."

As demonstrated below, a 20161127 build of gfortran 7.0.0 accepts code that
violates the above constraint:

$ cat poly-alloc-result.f90 
  type foo
  end type
  type(foo) :: bar
  bar = f()
contains
  pure function f() result(y)
    class(foo), allocatable :: y
    allocate(foo::y)
  end function
end 

$ gfortran poly-alloc-result.f90 

$ ./a.out

$ gfortran --version
GNU Fortran (MacPorts gcc7 7-20161127_0) 7.0.0 20161127 (experimental)