(j3.2006) Is 7.2.1.2p1(1) necessary?

[Van Snyder]

On Thu, 2015-03-12 at 13:03 -0700, Van Snyder wrote:
> Malcolm Cohen malcolm at nag-j.co.jp Wed Mar 11 22:44:07 MDT 2015 wrote:
> 
> >7.2.1.2p1(1) says "if the variable is polymorphic it shall be
> >allocatable and not a coarray."
> 
> Yes, this is necessary.
> 
> >Would it be enough
> >to require that if it is not allocatable, variable and expr shall have
> >the same dynamic type?
> 
> Not compile-time checkable.

If we made everything compile-time checkable, nobody would use Fortran.

> >So checking the
> >dynamic types, and raising an error condition if they're different,
> >isn't any more difficult that what's required now.
> 
> The whole point is to avoid raising error conditions since that crashes the 
> program!

If one knows for sure that the expression and the variable have the same
dynamic type, there wouldn't be a crash.

Otherwise, make the variable allocatable.

Without relaxing 7.2.1.2p1(1), how does one assign to an element of a
polymorphic array?  It seems that at least one level of SELECT TYPE is
necessary.

This is yet another of the unhelpful "helpful" restrictions about which
I wrote yesterday.  It's purported benefit can be gotten easily --
simply make the variable allocatable.  Its harm is greater than its
benefit:  Assignment to elements of polymorphic arrays is messy.