To: J3                                                     J3/23-185
From: T. Clune & subgroup generics
Subject: F202Y Use of kewords with deferred procedure arguments
Date: 2023-June-13

Reference:  23-155

Introduction
============

The introduction of REQUIREMENT/REQUIRES into the feature set for
generic programming has exposed an unfortunate issue that requires
resolution.  More specifically, the use of REQUIRES statements permits
multiple specifications of the interface of a deferred procedure whose
arguments all have consistent characteristics, and yet have different
argument _names_.  This then raises the issue of whether keyword
association can be used when referencing a deferred procedure.

Discussion
==========

Subgroup has considered various resolutions to this problem, and none
have universal support.  The existing syntax paper, 23-155, implicitly
disallows use of keywords in references to deferred procedures by
virtue of not defining the argument names for the "intersection" of
overlapping specifications.  This approach is backwards compatible
with most of the alternatives and was thus chosen as a safe baseline.

Alternative 1: Require all specifications of a deferred procedure to
use the same argument names.  This is problematic because we expect
individual requirements to developed independently in many instances.

Alternative 2: Allow keyword association when all specifications of a
deferred procedure, but otherwise disallow.   This is fragile, but at least
allows some developers to use keywords in this context.

Alternative 3: Provide some means to identify which specification
determines the argument names for keyword association.  E.g., if the
template has an explicit specification at the top level that could
serve as the "definitive" interface.  But we have also considered
adding an attribute to a REQUIRES or an INTERFACE that selects the
specification that provides the argument names.

Proposal
========

Subgroup would like to continue to evaluate alternatives to the
baseline syntax with regard to allowing use of keywords in deferred
procedure references inside templates.

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