J3/17-130
To:        J3 Members
From:      Van Snyder
Subject:   Polymorphic allocatable ultimate component of function result
Date: 2017 February 14

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

C1583 prohibits invoking a pure function in a context that might result
in executing an impure finalizer.  This is not constraint-able if a
function result has an allocatable polymorphic ultimate component, so
is a clear error in the standard.

C1584 prohibits a pure function from having a polymorphic allocatable
result.

C1586 prohibits an INTENT(OUT) dummy argument of a pure subroutine to be
such that invoking the subroutine might result in executing an impure
finalizer.

C1587 prohibits an INTENT(OUT) argument of a pure subroutine to be
polymorphic and allocatable, or to have a polymorphic allocatable
ultimate component.

C1583-C1584 are not completely parallel to C1586-C1587, in that C1587
prohibits a polymorphic allocatable ultimate component, and C1584 does
not.  Function results are very much like subroutines' INTENT(OUT) dummy
arguments, so they ought to have similar requirements.

To avoid the possibility of executing an impure finalizer as a
consequence of a reference to a pure function, and to be parallel to
C1587, C1584 also needs to prohibit a polymorphic allocatable ultimate
component.

This is a technical change from Fortran 2008.

2. Edits
========

[31:1+ 4.3.3 Fortran 2008 compatibility] Insert a paragraph:

"Fortran 2008 permitted a pure function to have a result that has a
polymorphic allocatable ultimate component; this document does not
permit that."

[336:25 C1584 15.7 Pure procedures]
   Replace whole constraint "A pure ... result" with
"C1584 The function result of a pure function shall not be both
       polymorphic and allocatable, or have a polymorphic allocatable
       ultimate component."
{Compare with C1583 and C1587.}