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From: "Randy Brukardt" <randy@rrsoftware.com>
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Subject: Re: Aggregate with derived types.
Date: Sat, 16 Sep 2023 01:39:57 -0500
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"Blady" <p.p11@orange.fr> wrote in message 
news:udvooj$2oveh$1@dont-email.me...
...
> I wonder why the float list aggregate isn't inferred by the compiler and 
> need some help with a qualification.

The language rule is that the ancestor_part of an extension aggregate is 
expected to be of "any tagged type" (see 4.3.2(4/2)). An aggregate needs to 
have a single specific type, and "any tagged type" is not that.

The reason that the ancestor is "any tagged type" is that the type of the 
ancestor determines the extension components needed along with other 
legality rules. One could imagine a language where all of these things are 
decided simulateously, but people worried that the complexity would make it 
difficult/impossible to implement. So aggregates are essentially black boxes 
whose type has to be determinable from the outside, and similar rules exist 
for parts inside the aggregate.

                                    Randy.