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From: "Jeffrey R.Carter" <spam.jrcarter.not@spam.acm.org.not>
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Subject: Formal Package Compiler Differences
Date: Fri, 14 Jul 2023 21:17:13 +0200
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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