J3/11-194
To: J3
From: Malcolm Cohen
Subject: Multiple choice non-polymorphic allocation interp.
Date: 2011 June 13

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NUMBER:
TITLE: Non-polymorphic ALLOCATE with polymorphic SOURCE=
KEYWORDS: ALLOCATE, polymorphic, SOURCE=.
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Consider
  Program m195_m1
    Type t
      Real c
    End Type
    Type,Extends(t) :: t2
      Real d
    End Type
    Class(t),Allocatable :: x
    Type(t),Allocatable :: y
    Allocate(x,Source=t2(1.5,-1.5))
    Allocate(y,Source=x)               ! (*)
    ...
  End Program

Is the second ALLOCATE statement (marked *) standard-conforming?

The only requirement is that Y be type-compatible with X; they both
have the same declared type so this is true.

However, the very similar
  Allocate(y,Source=t2(1.2,-1.5))
would not be conforming because Y is not type-compatible with the
structure constructor, and
  y = x
would also also not conforming (the dynamic types being different).

However, the standard says that
  "the value of [y] becomes that of [x]"
which is clearly impossible.

Since the standard fails to establish an interpretation one might
conclude that the example is not conforming.  However, the similar
situation with the dynamic types being the same but with non-deferred
length type parameters being different is explicitly stated to be
conforming (and to raise an error condition), perhaps this was also
intended to raise an error condition.

It is also possible that the intent was to use the value of the
declared type part in this case, as happens for pointer assignment.

What is the interpretation of this ALLOCATE statement?

ANSWER 1:

The statement was intended to be conforming and to use the declared
type part of the source-expr only.  An edit is supplied to clarify.

ANSWER 2:

The statement was intended to be conforming and to cause an error
condition at runtime if the dynamic types were different: an edit is
supplied to clarify.

ANSWER 3:

The statement was not intended to be conforming.  An edit is supplied
to clarify.

EDIT 1:

[128:24] In 6.7.1.2p7, before "On successful", insert
  "If an <allocate-object> is not polymorphic and the <source-expr> is
   polymorphic with a dynamic type that differs from its declared
   type, the value provided for that <allocate-object> is the ancestor
   component of the <source-expr> that has the type of the
   <allocate-object>; otherwise, the value provided is the value of
   the <source-expr>."
[128:25-26] Replace "that of <source-expr>" with "the value provided",
            twice.

EDIT 2:

[128:24] In 6.7.1.2p7, before "On successful", insert
  "If an <allocate-object> is not polymorphic and the <source-expr> is
   polymorphic with a dynamic type that differs from its declared
   type, an error condition occurs."

EDIT 3:

[128:24] In 6.7.1.2p7, before "On successful", insert
  "If an <allocate-object> is not polymorphic, the dynamic type of the
   <source-expr> shall not differ from its declared type."

SUBMITTED BY: Malcolm Cohen

HISTORY: 11-nnn   m195 Submitted

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