Okay, so after reading through the linked issue here: 
https://dev.clojure.org/jira/browse/CLJ-322 I'm not sure, as a tool 
builder, what is the ideal path forward.

This is what I seem to understand would be ideal, let me know if I'm wrong:

1) AOT compile everything. So basically, always AOT every lib. This is to 
make sure that we don't face this issue: 
https://dev.clojure.org/jira/browse/CLJ-1544

2) Figure out some mechanism within the built tooling to only include the 
classes for which the source exist in the current project being built into 
the resulting Jar. Meaning, only namespaces found in the source folder 
should have their compiled classes be included into the Jar.

This setup should minimize conflicts, from what I understand. The only 
thing I'm not sure about is, are classes compiled with an older version of 
Clojure and JDK always going to work with a newer version of Clojure and 
JDK?

On Friday, 25 January 2019 22:11:46 UTC-8, Didier wrote:
>
> So I got to the bottom bottom of the problem here.
>
> This is a scenario:
>
> 1) Library A depends on library B and Clojure 1.10
> 2) Library B must be AOTed due to a gen-class, but depends on Clojure 1.9
> 3) Library A does not work, because it is now using the Clojure core spec 
> 1.9 compiled transitively through the Library B AOT, and found in its Jar, 
> since .class get precedence over source files.
>
> So, this means that a library with a dependency on another one that is AOT 
> can cause spec conflicts.
>
> This is new now that spec is out. And so I first caught this when some of 
> our libs were using 1.9, and I started moving others to 1.10.
>
> The way I solved this is by hacking our build, so that the clojure 
> compiled classes from the AOT don't get included in the Jar after they are 
> compiled.
>
> In my opinion, this is a bit of a problem. What would be nice is either to 
> have a way to compile only a gen-class, and not the namespace that contains 
> it. Or not compile things transitively. Or maybe just a way for compile to 
> exclude clojure core namespaces.
>
> Now, if you depend on libraries that don't need AOT, it is not an issue. 
> But if you do, it forces you to re-compile all your dependencies and do a 
> full big bang upgrade to the new Clojure version. You can't just use libs 
> compiled with older versions of spec. While before, you were able to use 
> libs compiled with older version of Clojure from packages that use newer 
> version of Clojure.
>
> On Wednesday, 16 January 2019 20:47:43 UTC-8, Mark Derricutt wrote:
>>
>> On 16 Jan 2019, at 18:17, Alex Miller wrote:
>>
>> Yes, it's one of the downsides of AOT. 
>>
>> I'd say it's not so much a downside of AOT - but of having a single 
>> output path for classes. I've long wanted Chas's patch to be applied, or 
>> something like it.
>>
>> Having everyone reinvent the mechanism whenever they happen to need AOT 
>> is kind of annoying - rare, but still annoying.
>> ------------------------------
>>
>> "The ease with which a change can be implemented has no relevance at all 
>> to whether it is the right change for the (Java) Platform for all time." — 
>> Mark Reinhold.
>>
>> Mark Derricutt
>> http://www.theoryinpractice.net
>> http://www.chaliceofblood.net
>> http://plus.google.com/+MarkDerricutt
>> http://twitter.com/talios
>> http://facebook.com/mderricutt
>>
>

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