J3/17-200
To: J3
From: Salvatore Filippone & Damian Rouson
Subject: Constraint C749
Date: 2017 September 26

1. Discussion
=============

In the Parallel Sparse Basic Linear Algebra Subroutines (PSBLAS) library,
communication buffers would ideally be coarray components inside derived
types that have other components for storing the data prior to packing the
data into the buffer. Storing the buffer in the same derived type as the
data that will be packed into the buffer clarifies the data dependencies,
making it less likely that a programmer will rely on critical sections to
manage buffer accesses.  A Fortran 2015 implementation of PSBLAS would
have data structures of the following form:

  type vector
    real, allocatable :: component(:),component_buffer(:)[:]
  end type

  type(vector) :: bundle, field

This fixes the number of such data objects in the program (two in the above
case: bundle and field).  Fixing the number of data objects at compile time
is undesirable in the context of a library  designed to be installed and
used  in multiple applications, and because it prevents those same objects
from being reused as components of other higher-level objects in a
sufficiently flexible way.

However, C749 in N2137 states,

   "A data component whose type has a coarray ultimate component shall be a
   nonpointer nonallocatable scalar and shall not be a coarray,"

which precludes the more flexible data structures required in PSBLAS:

  type(vector), allocatable :: bundle(:) ! Violates C749 for above vector
  allocate(bundle(100))


2. Edits
========

When a base document for Fortran 2020 has been provided, the text
corresponding to C749 in N2137 should be edited to insert "nonallocatable"
before "coarray ultimate component."  This will have the effect of allowing
allocatable arrays of a derived type with an allocatable ultimate coarray
component.  It will still disallow nonallocatable coarray ultimate
components, which avoids unnecessary synchronizations upon allocation of
the container object.