To: J3                                                     J3/23-185r1
From: T. Clune & subgroup generics
Subject: F202Y Use of keywords with deferred procedure arguments
Date: 2023-June-13

Reference:  23-155, 23-163

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: One simple option is to require the programmer to
specify an explicit interface (in the specification part of the
template) for any deferred procedure that they wish to invoke with
keyword arguments.

Alternative 2: Require all specifications of a deferred procedure to
use the same argument names.  This is problematic because we expect
individual requirements to be developed independently in many
instances.

Alternative 3: 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 4: A final alternative is to add a new keyword/statement
that the programmer can use to select one of the interfaces as the one
that specifies the keywords.

Paper 23-163 provides a more extensive list of pros and cons for the
alternatives as well as concrete examples.

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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