Indefinite_Holders: bug or misunderstanding?

["Alejandro R. Mosteo" <alejandro@mosteo.com>]

I have used the Indefinite_Holders sometimes to make definite types from 
classwide ones, or variant records without defaults. However this sample 
case doesn't work with an array (with GNAT 2017 or GCC 7.2.0) and I'm 
unsure wether it should:

---8<---

with Ada.Containers.Indefinite_Holders;

procedure Test_Holder is

    package Inner is

       type Str (<>) is private;

    private

       type Str is array (Positive range <>) of Character;

       -- type Str (Len : Natural) is record
       --    S : String (1 .. Len);
       -- end record;

    end Inner;

    package Str_Holders is new Ada.Containers.Indefinite_Holders
       (Inner.Str, Inner."=");

begin
    null;
end Test_Holder;

---8<---

The commented alternate Str type does compile fine. The error I'm 
getting for the uncommented one is:

$ gnatmake test_holder.adb
gcc -c test_holder.adb
test_holder.adb:19:04: instantiation error at a-coinho.adb:337
test_holder.adb:19:04: invalid constraint: type has no discriminant
test_holder.adb:19:04: instantiation error at a-coinho.adb:337
test_holder.adb:19:04: unconstrained subtype not allowed (need 
initialization)
test_holder.adb:19:04: provide initial value or explicit array bounds
test_holder.adb:19:04: instantiation error at a-coinho.adb:337
test_holder.adb:19:04: expected private type "Str" defined at line 7
test_holder.adb:19:04: found type "Str" defined at line 11
gnatmake: "test_holder.adb" compilation error

Something obvious I'm missing?

Thanks,
Alex.