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From: "Alejandro R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada Successor Language
Date: Mon, 25 Jun 2018 16:18:47 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:53300
Date: 2018-06-25T16:18:47+02:00
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On 25/06/2018 14:41, J-P. Rosen wrote:
> Le 25/06/2018 à 10:58, Alejandro R. Mosteo a écrit :
>> See this one, which is the case that bit me recently:
>>
>> procedure Blah is
>>      generic
>>          type Something is private;
>>      package Duh is
>>          type Meh is new Something;
>>      end Duh;
>>
>>      type Oh is tagged null record;
>>      
>>      package Ouch is new Duh (Oh);
>>
>> begin
>>     null;
>> end Blah;
> 
> A private formal allows tagged and untagged types. However, deriving
> without extension is legal only for untagged types. Since it is a
> specification (that you are supposed to have read), the compiler assumes
> the best and allows the derivation. When you instantiate with a tagged
> type, you would be doing something illegal and the instantiation is not
> allowed.

Someone explained it to me at the time, and your best/worst explanation 
is great. It was rather an example of that you can have a generic that 
compiles, a type that fulfills the generic formal, and an instantiation 
that fails, which admittedly irks me.

> The same derivation would not compile in the body of the generic, where
> the worst is assumed
GNAT 7.3/2018 does compile it if I move it to the private part or body, 
which thus must be a bug, as hinted by that it allows taking the type's 
'Class...

procedure Blah is

generic
	type Something is private;
package Duh is					
private	
	type Meh is new Something;
	function Yes (M : Meh) return Boolean is (True);
	function Yes_Class (M : Meh'Class) return Boolean is (True);
end Duh;

type Oh is tagged null record;

package Ouch is new Duh (Oh); -- This fails with non-tagged formal

begin
    null;
end Blah;