11-124r1
To: J3
From: Malcolm Cohen
Subject: Interop: On assumed type
Date: 2011 February 15


1. Introduction

For BIND(C) procedures, an assumed-type array that is neither assumed-shape
nor assumed-rank must be passed as a simple void * pointer, i.e. the same
as TYPE(C_PTR).

This means that the array element size or indeed any other information
about its type will not be directly available.  Without this information an
assumed-shape dope vector, or array pointer dope vector, or C descriptor
cannot be constructed.

Therefore it is proposed that it be forbidden to pass such an array as an
actual argument corresponding to an assumed-shape or assumed-rank dummy.
(The pointer association case is already covered.)

See 11-124 for an example.

Also, the TR provides little guidance as to what the contents of the type
field of the C descriptor should contain in various cases.


2. Discussion

2.1p2 is problematic, since it implies that CLASS(*) is assumed-type.
It is also very strange to read that an assumed-type object "is" something
followed by a constraint that it "shall be" something.
An edit is provided.

Review of 2.1p3 showed that it was much too restrictive, disallowing
TYPE(*) entities from appearing in type declaration statements etc..
(Are these meant to be only implicitly typed?)


3. Open technical issues and suggested straw votes

(1) Why is explicit-shape assumed-type allowed?  One is not permitted to
    access array elements or take sections etc. so this seems useless.

Straw vote 1:
  (a) do not allow assumed-type to have explicit shape,
  (b) ditto, and require assumed-type assumed-size to have rank 1,
  (c) remain as is,
  (d) undecided.
?

(2) Why does the type member in the C descriptor even exist?  Existing
    C libraries require type to be passed separately already.

(3) Even if type member exists, why does it require the processor to tell
    the difference between an interoperable struct and a non-interoperable
    one?  Even CLASS(*) doesn't require this!  How does it help the user
    to know that he has something that is interoperable but of otherwise
    completely unknown type?

Straw vote 2: The type member should
  (a) not exist
  (b) not treat interoperable structs differently
  (c) remain as is
  (d) undecided
?


3. Edits to N1838

[3:10-12] 2.1p2, replace the whole paragraph with
  "An entity declared with a <declaration-type-spec> of TYPE (*) is an
   assumed-type entity.  It has no declared type and its dynamic type and
   type parameters are assumed from its effective argument."

[3:15] 2.1p3, after "An assumed-type variable shall"
       replace "appear ... with"
       with "not appear in a designator or expression except as an
             actual argument corresponding to".
{It is only its appearance in/as a designator/expression that is
 problematic.}

[3:17] Append to 2.1p3
  "An assumed-type array that is explicit shape or assumed size
   shall not appear as an actual argument corresponding to an
   assumed-shape or assumed-rank dummy argument.".
{This just modifies the requirements of the existing 2.1p3, so should
 not be a constraint unless we want to make the whole 2.1p3 into a
 constraint.}

[5:6] "is also required to" -> "shall".
{Remove repetitive redundancy, use correct ISO language.}

[17:14] 5.2.8p5, at the end before the final full stop, insert
  "; the type member shall have a value from Table 5.2 that depends on the
   effective argument as follows:
   (-) if the effective argument is TYPE(*), CFI_type_unspecified;
   (-) if the effective argument is CLASS(*) with a dynamic type that is an
       interoperable intrinsic type, the appropriate value for that type;
   (-) if the effective argument is of an intrinsic interoperable type, the
       appropriate value from the table;
   (-) if the effective argument is not polymorphic and is of an
       interoperable derived type, CFI_type_struct;
   (-) otherwise, CFI_type_unspecified.".
{Bullet list.
 I think that polymorphic actual arguments are forbidden except for TYPE(*)
 and CLASS(*), so this covers all the cases.}

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