Yet another gnat bug

[George Shapovalov <gshapovalov@gmail.com>]

This will probably sound more like venting frustration. Sorry if so. But how does anybody get anything done? 
gnat is *the major* Ada compiler and pretty much the only one implementing the standard in full. Yet I cannot seem to get it working past really small size in any project. As soon as I try to get any basic type composition done (only 3-4 inheritance levels, with, perhaps double interface overlay), I get that dreaded gnat bug message.. This is at least the 3rd one just within past week or two..
Specifically this:
https://github.com/gerr135/wann/tree/gnat_bug01
(the bug triggering code is in a separate branch pointed to by that link).

This is still early in design phase and far from being functional in any way, so I don't really expect much comments on the code itself (thus that "venting frustration" comment above). But the pattern that seems to universally trigger these gnat bugs is something along these lines:

type Base_Interface is interface;
..

type Derived1_Interface is new Base_Interface and ..;
..
perhaps few more layers here..

then 
type Base_impl1 is new Base_Interface with private;
..
type Derived1 is new Base_impl1 and Derived1_Interface with private..

basically trying to stitch together functional interface hierarchy (containing algorithmic stuff) and data storage type hierarchy. Somehow gnat very often just cannot handle this type of design :(.
(and yes, I am avoiding having to lay generics on top of other generics like Dmitry suggests - keeps design and compilation times sane, but apparently overloads gnat capacity to deal with abstraction).

So, I guess my question would be - how people deal with such situations (combining algorithmic and data representation type hierarchies) in their experience? Or, whether too many child modules makes any difference? I seem to have noticed that the more hierarchical my packages are (but this one is only like 3rd level child!) the more often I trigger that gnat bug message..
(but then keeping the code in one huge module is really messy too!)

And yeah, the specific message here is:
gprbuild -P wann.gpr
Compile
   [Ada]          run_customnn.adb
+===========================GNAT BUG DETECTED==============================+
| Community 2018 (20180524-73) (x86_64-pc-linux-gnu) GCC error:            |
| in gnat_to_gnu_entity, at ada/gcc-interface/decl.c:429                   |
| Error detected at wann-nets-vectors.ads:106:5 [run_customnn.adb:23:5]    |
| Please submit a bug report by email to report@adacore.com.               |
| GAP members can alternatively use GNAT Tracker:                          |
| http://www.adacore.com/ section 'send a report'.                         |
| See gnatinfo.txt for full info on procedure for submitting bugs.         |
| Use a subject line meaningful to you and us to track the bug.            |
| Include the entire contents of this bug box in the report.               |
| Include the exact command that you entered.                              |
| Also include sources listed below.                                       |
| Use plain ASCII or MIME attachment(s).                                   |
+==========================================================================+

and the "please include" list of files lists pretty much all of them in the src dir.

But as I said, this is rather a pattern I observe, not just one-off situation..
This is with the latest FSF gnat compiler (2018 release based on gcc-7.3.1 backend, Gentoo Linux, relatively fresh everything else).

Sigh, I guess another report to file with AdaCore..
Sorry for disturbance here..