Re: Discriminants of tagged types

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Maciej Sobczak <see.my.homepage@gmail.com> writes:

> GNAT says that discriminants of tagged types cannot have default
> values.

Well, Ada says that.  GNAT is just following orders.  ;-)

> but this is an error:
>
>    type T (A : Integer := 0) is tagged null record;

We tried very hard during Ada 9X to allow this, but
we kept running into semantic difficulties, which required
more and more complicated rules to fix.  One day,
Tucker said to me (or I said to Tucker -- I don't
remember which), let's just outlaw this.  We both
agreed it was a big simplification.

Sorry, I don't remember in detail what the semantic
difficulties were.  Ludovic's explanation seems
as good as any.

I believe Ada 2012 will allow defaults for LIMITED
tagged types, and I think (not sure) it's already
implemented in GNAT under -gnat2012 mode.  The limited case
is easy, because the weird rule that says "defaulted
discriminants can change" isn't true for limited
types, because assignment statements are forbidden.

- Bob