Rejected (legal?) program

[Nick Foubert <nick@metropolitan.ai>]

I raised this in the ada-lang gitter channel and it was suggested that this is likely a bug, but figured I'd post here before raising a bug report just in case.

The following example program gives me a compile error with x86-64 gnat 12.2 (can also be seen at https://ada.godbolt.org/z/45G1x9sEd)

example.adb:70:05: error: instantiation error at line 67
example.adb:70:05: error: "P_Formal_Type" is not a visible entity of "P_Instance"

-----

procedure Example is

    generic
        type E_Formal_Type is 
            abstract tagged private;    
    package E is end;

    type E_Actual_Type is tagged null record;

    package E_Instance is new E (E_Actual_Type);

    generic
        with package E_Formal_Pkg is new E (<>);

        type D_Formal_Type is 
            new E_Formal_Pkg.E_Formal_Type with private;

    package D is end;

    type D_Actual_Type is 
        new E_Actual_Type with null record;

    package D_Instance is 
        new D (E_Instance, D_Actual_Type);

    generic    

        with package E_Formal_Pkg is new E (<>);

        with package D_Formal_Pkg is 
            new D
                (E_Formal_Pkg => E_Formal_Pkg,
                others           => <>);

        type P_Formal_Type is 
            new E_Formal_Pkg.E_Formal_Type with private;
            
    package P is end;

    type P_Actual_Type is 
        new E_Actual_Type with null record;

    package P_Instance is 
        new P (E_Instance, D_Instance, P_Actual_Type);

    generic
        with package E_Formal_Pkg is new E (<>);

        with package D_Formal_Pkg is 
            new D (E_Formal_Pkg => E_Formal_Pkg,
                   others       => <>);

        with package P_Formal_Pkg is 
            new P
                (E_Formal_Pkg => E_Formal_Pkg,
                 D_Formal_Pkg => D_Formal_Pkg,
                 others       => <>);

    package Test is 
        type Test_Type is 
            new P_Formal_Pkg.P_Formal_Type with null record; -- Line 67
    end Test;

    package Test_Instance is -- Line 70
       new Test (E_Instance, D_Instance, P_Instance);
        

begin
    null;
end Example;

---

Per ARM 12.7.10/2 I believe that should be legal? 

A slight modification as can be seen at https://ada.godbolt.org/z/4x4h4s7f5 compiles successfully.