To: J3                                                     J3/20-127
From: Malcolm Cohen
Subject: Public namelist and private variable
Date: 2020-February-27

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NUMBER: F18/018
TITLE: Public namelist and private variable
KEYWORDS: NAMELIST PUBLIC PRIVATE
DEFECT TYPE: Clarification
STATUS: J3 consideration in progress

QUESTION:

Consider

  Module m1
    Real,Public :: x
  End Module
  Module m2
    Use m1
    Private x
    Namelist/nml/x
  End Module

On the face of it, module M2 appears to violate
  C8105 (R868) A namelist-group-object shall not have the PRIVATE
       attribute if the namelist-group-name has the PUBLIC attribute.
as the local X indeed has the PRIVATE attribute. On the other hand,
it is just a local name for the remote X which is PUBLIC, which
raises doubts.

Comment: This seems to be a very old constraint dating back to when
         the standard was much more restrictive about such things.
         It is not clear why this should be disallowed.
         Even if X were a local variable of M2, it is not clear what
         purpose this constraint serves.

A quick compiler survey revealed that most but not all compilers
think that it is where the variable is defined that matters, i.e.
many accept the example code.

Q1. Should PRIVATE local variables really be prohibited from a PUBLIC
    namelist?

If the answer to Q1 is yes,
Q2. Is it PRIVATE on the local name that matters, or PRIVATE on the
    variable where it is defined?

COMMENT:

A NAMELIST statement can contain several namelist-group-names, so it
is also somewhat ambiguous as to which namelist-group-objects this
constraint applies to.

ANSWER:

A1. Yes. Although this restriction appears to serve little purpose,
    it is a deliberate restriction (which could be lifted in a future
    standard).

A2. It is whether the variable is defined to have the PRIVATE
    attribute that matters, not the local name.

ALTERNATIVE
A2. It is whether the local name has the PRIVATE attribute that
    matters, not where the variable is defined.
END ALTERNATIVE

An edit is provided.

EDIT to 18-007r1:

[119:8-9] 8.9 NAMELIST statement, C8105,
  After "PRIVATE attribute"
  insert "in the scope where the variable is defined"
  and change "the namelist-group-name" to "its namelist-group-name",
  making the whole constraint read
    "C8105 (R868) A namelist-group-object shall not have the PRIVATE
           attribute in the scope where the variable is defined if
           its namelist-group-name has the PUBLIC attribute."

ALTERNATIVE
[119:8-9] 8.9 NAMELIST statement, C8105,
  After "PRIVATE attribute" insert "in the local scope",
  and change "the namelist-group-name" to "its namelist-group-name",
  making the whole constraint read
    "C8105 (R868) A namelist-group-object shall not have the PRIVATE
           attribute in the local scope if its namelist-group-name
           has the PUBLIC attribute."
END ALTERNATIVE

{COMMENT TO J3: The constraint should probably be re-wordsmithed to be
 more concise and easier to read, perhaps by beginning
   "If a namelist-group-name has the PUBLIC attribute..."
 but this wordsmithing can be considered at a later date.}

SUBMITTED BY: Malcolm Cohen

HISTORY: 20-nnn   m221  F18/018 Submitted

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