To: J3                                                     J3/21-123
From: Malcolm Cohen
Subject: Interp F18/019 PURE and default initialization
Reference: 20-151
Date: 2021-February-28

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NUMBER: F18/019
TITLE: PURE and default initialization
DEFECT TYPE: Erratum
STATUS: J3 consideration in program

QUESTION:

An essential property of pure procedures is that they do not modify
the values of nonlocal variables except through dummy arguments. The
addition of default initialization of pointer components makes it
possible to violate this property. Because default initialization does
not imply the SAVE attribute, a local variable of derived type in a
pure procedure can include a pointer component whose target is a
nonlocal variable. The definition of pure procedures in subclause 15.7
allows a program to modify the value of a nonlocal variable through
such a component.

For example,

    PROGRAM EXAMPLE1
      REAL, TARGET :: X = 1.0
      TYPE T
        REAL, POINTER :: P => X
      END TYPE T
      CALL SUBR
      PRINT *, X ! X has been changed to 2.0
    CONTAINS
      PURE SUBROUTINE SUBR
        TYPE(T) Y
        Y%P = 2.0
      END SUBROUTINE SUBR
    END

A local variable is not needed, for example,

    PROGRAM example2
      REAL,TARGET :: x = 123
      TYPE t
        REAL,POINTER :: p => x
      END TYPE
      CALL sub
      PRINT *,x ! No longer == 123?
    CONTAINS
      PURE SUBROUTINE sub()
        ASSOCIATE(y=>t())
          y%p = -999 ! Affects x.
        END ASSOCIATE
      END SUBROUTINE
    END PROGRAM

A polymorphic variable can be used, for example,

    PROGRAM example2
      REAL,TARGET :: x = 123
      TYPE t
      END TYPE
      TYPE,EXTENDS(t0) :: t
        REAL,POINTER :: p => x
      END TYPE
      CALL sub
      PRINT *,x ! No longer == 123?
    CONTAINS
      PURE SUBROUTINE sub()
        CLASS(t0) y ! Declared type has no initialized ptr comp.
        ALLOCATE(t::y) ! Without SOURCE=, gets a pointer to x.
        SELECT TYPE(y)
        TYPE IS (t)
          y%p = -999 ! Affects x.
        END SELECT
      END SUBROUTINE
    END PROGRAM

ALLOCATE with MOLD= instead of a type-spec can do the same.

If component initialization is not an attribute of the component, more
convoluted examples are possible using SEQUENCE types so that the
local variable does not have default initialization but can be
initialized using a compatible type that does.

Q. Was it intended to allow nonlocal variables to be modified by a
   pure procedure in this way?

ANSWER:

A: No. An edit is supplied to correct the standard.

Note: The question of whether component initialization is an attribute
      should be addressed by a separate interpretation request.

EDIT to 18-007r1:

RECOMMENDED EDIT:
[324:20-] 15.7 Pure procedures, between NOTE 1 and C1590,
          insert new constraint
   "C1589a A named local entity or construct entity of a pure
           subprogram shall not be of a type that has default
           initialization of a pointer component to a target at
           any level of component selection."
{With no local or construct entity designator of a "bad" type being
 allowed, one cannot write a pointer component reference to it.
 This is stricter than strictly necessary, as it effectively forbids
 such types from any usage within pure, even unproblematic usage.}

BEST EDIT:
[324:28] 15.7 Pure procedures,
 Change C1594 after "is a coindexed object,"
            insert "has a <part-ref> that is a pointer component that is
                    default-initialized to an <initial-data-target>,"
{Attempt to prohibit all the bad usages, while leaving the good ones.
 Additional edits will be needed for SIMPLE procedures.
 This makes a complicated constraint more complicated, and may be more
 work to implement than the recommended edit.}

ALTERNATIVE EDIT:
    Forbid the default initialization of data pointer components to a
    target other than NULL().
{This is even more strict. Default initialization of data pointer
 components to a target is a pretty arcane feature though.}

SUBMITTED BY: Robert Corbett

HISTORY: 20-151  m222  Submitted
         21-nnn  m223  Revised with answer

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