To: J3                                                     J3/25-121
From: Malcolm Cohen
Subject: Corank and genericity
Date: 2025-February-18
Reference: 25-103


1. Discussion

Paper 25-103 correctly observes that
    "Coranks play no part in generic resolution"

This may have been a mistake. Corank works the same way as rank does in
most situations: e.g. for bounds, extents, and subscripting we have
cobounds, coextents, and cosubscripting.

It is long past the time when this part of coarray design could be changed
very much, however, 25-103 continues on to say
    "because the coranks of an actual argument and the corresponding dummy
     argument are not required to agree"

This is a specious argument:
    Rank agreement is not required for explicit-shape.
    Corank agreement is not required for explicit-coshape.
These are almost identical situations.

In fact it is an incorrect argument:
    For deferred coshape, the coranks are in fact required to agree,
    just as they are for deferred shape.
    (See [351:18] 15.5.2.7 Allocatable dummy variables, p3.)

Consider:

    GENERIC :: g => p
    REAL,ALLOCATABLE :: x[:,:,:]
    CALL g(x)

    SUBROUTINE p(y)
        REAL,ALLOCATABLE :: y[:,:]
        ALLOCATE(y[100,*])
        ...

Generic resolution turns "CALL g" into "CALL p", which is then incorrect
because the coranks differ.

Worse, we cannot have a generic that can take corank 2 allocatable and
corank 3 allocatable unless we mangle the argument lists to make the calls
incompatible (and thus very confusing!). This is because, as 25-103 quite
rightly points out, corank is not considered during generic resolution.


2. Proposal

Corank should be considered as a disambiguator when one dummy argument is
an allocatable coarray and the other dummy argument is an allocatable
coarray of different corank.

(This is somewhat analogous to how allocatable and pointer are considered
to be disambiguators with respect to each other, though they are not with
respect to non-allocatable and non-pointer.)


3. Comment

This is not sufficient to make auto-generic handle corank genericity, as
although we added a RANK(value) clause to the type declaration statement,
we neglected to add a CORANK(value) clause to it.

===END===