J3/16-289
To: J3
From: Malcolm Cohen
Subject: Pure statement functions are too volatile
Date: 2016 October 12

1. Problems

Consider the following module:

Module m
  Use Iso_C_Binding
  Integer(C_int),Volatile :: vcount
Contains
  Subroutine s(x)
    Integer i,pure_stmtfn,x(:)
    pure_stmtfn() = vcount
    Forall (i=1:Size(x)) x(i) = pure_stmtfn()
  End Subroutine
End Module

Statement function pure_stmtfn is pure because it references no
impure function.  However, vcount may change at any time so this is
not actually pure.  This was fixed for normal pure procedures by
Interp F03/0126, which prohibited the designator of a VOLATILE
variable from appearing within a pure subprogram.  However, it did
not address statement functions.

Also, it appears that the following is valid, as long as the variable Z is
not used.

Pure Subroutine strange(y)
  Integer,Intent(Out) :: y
  Integer,Volatile :: z
  y = 0
End Subroutine

Permitting Z to be declared, but not used, within STRANGE is strange.


2. Solution

Therefore we should do the same as F03/0126 for statement functions.
This will be a new incompatibility with Fortran 2003/8.

We should also fix this gratuitous strangeness with local volatiles.
It will also be a new incompatibility with Fortran 2003/8.

Note that we could address this via the interpretation process, but
given the timing, and especially since statement functions are
obsolescent, it seems simpler just to make the change to F2015.


3. Edits to 16-007r2

[30:30+] 4.3.3 Fortran 2008 compatibility, after p3,
         insert new paragraph
  "Fortran 2008 permitted a pure statement function to reference a
   volatile variable, and permitted a local variable of a pure
   subprogram or of a BLOCK construct within a pure subprogram to be
   volatile (provided it was not used); this document does not permit
   that."
{Incompatibility with Fortran 2008.}

[31:21+] 4.3.4 Fortran 2003 compatibility, after p9,
         insert new paragraph
  "Fortran 2003 permitted a pure statement function to reference a
   volatile variable, and permitted a local variable of a pure
   subprogram to be volatile (provided it was not used); this document
   does not permit that.
{Incompatibility with Fortran 2003.}

[336:14] 15.7 Pure procedures, p1,
  After "only pure functions"
  insert "and does not contain the <designator> of a variable with
          the VOLATILE attribute" (all in obsolescent font).
{Prohibit pure statement function from referencing a volatile variable.}

[336:29] same subclause, C1593,
         between "not have the SAVE" and "attribute"
         insert "or VOLATILE".
{Prohibit a local volatile variable in a pure subprogram.}

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