Formal Package Compiler Differences

[Jeffrey R.Carter]

I have a generic pkg;

with Ada.Containers.Vectors;

generic -- Vector_Conversions
type Index is range <>;
type Element is private;
type Fixed is array (Index range <>) of Element;

with function "=" (Left : in Element; Right : in Element) return Boolean
is <>;

with package Unbounded is new Ada.Containers.Vectors
(Index_Type => Index, Element_Type => Element);
package Vector_Conversions is
-- Empty
end Vector_Conversions;

and an instantiation of

with Ada.Containers.Vectors;
with Ada.Strings.Unbounded;
with Vector_Conversions;

procedure Instance_Test is
use Ada.Strings.Unbounded;

package String_Lists is new Ada.Containers.Vectors
(Index_Type => Positive, Element_Type => Unbounded_String);

type String_List is array (Positive range <>) of Unbounded_String;

package Conversions is new Vector_Conversions
(Index => Positive,
Element => Unbounded_String,
Fixed => String_List,
Unbounded => String_Lists);
begin -- Instance_Test
null;
end Instance_Test;

Compiler G compiles this and it works fine.

Compiler O fails to compile the instantiation with the error msg "The subprogram
actuals for the formal package and actual package must statically denote the
same entity", pointing to the Unbounded parameter association and referencing
ARM 12.7(8). If I add an explicit "=" with "is <>" to Vector_Conversions, then
compiler O compiles it and it works fine.

Possibly one of the compilers has an error, but possibly they're both correct,
and I'm wondering if it's possible to tell which is the case.

(If anyone is interested, the actual generic pkg where I encountered this is
PragmARC.Conversions.Vectors from the PragmAda Reusable Components
[https://github.com/jrcarter/PragmARC]), and I will supply the actual code that
instantiates it on demand. What I've presented is a first attempt at a minimal
reproducer.)

--
Jeff Carter
"Make sure your code 'does nothing' gracefully."
Elements of Programming Style
196

[Jeffrey R.Carter]

On 2023-07-14 21:17, Jeffrey R.Carter wrote:
> I have a generic pkg;
>
> with Ada.Containers.Vectors;
>
> generic -- Vector_Conversions
>    type Index is range <>;
>    type Element is private;
>    type Fixed is array (Index range <>) of Element;
>
>    with function "=" (Left : in Element; Right : in Element) return Boolean
>    is <>;

Oops! This "=" should be removed to get the behavior described.