I have encountered this problem as well. Did not verify the explanation, but sounds reasonable.

Clojure 1.3 om Mac OS 10.7, Clojure 1.5.0 alpha1 on Linux x86_64 (OpenJDK 1.7.0 b147)

The attached patch fixes the issue, by emitting IPersistentMap instead of Persistent{Hash|Array}Map as class type for maps literals

I uploaded another patch fixing the same problem in a different way.
While 0001-Fix-for-CLJ-944.patch makes clojure.lang.Complier.ConstantExpr#getJavaClass return clojure.lang.IPersistentMap for both clojure.lang.PersistentHashMap and clojure.lang.PersistentArrayMap, 0002-Fix-for-CLJ-944.patch makes clojure.lang.Compiler.MapExpr#parse return a PersistentArrayMap if the length is <= HASHTABLE_THRESHOLD, instead of always returning a PersistentHashMap.

This approach is more consistent, making the type of the compiler's internal representation of a map literal equal to the one of the reader.

Note that this second approach while being more consistent, breaks some tests that assume some operations on maps (specifically `seq` and `print`) to be order dependent, and written with the hash-map return order implementation in mind.

That should not be the case and if the second patch is preferred over the first one, I'll gladly fix those tests.

Approach #2, relying on consistent choice of concrete map class by size throughout, feels quite fragile.

Approach #1 seems to abuse the method name getJavaClass(), now having it return "get the base type I would need for cast".

Maybe there needs to be a different thing entirely?

Patch #2 should get merged (IMHO) regardless of the fragility of its approach to fixing this ticket's bug, since it fixes another bug:

prior to the patch:

user=> (class {:a 1})
clojure.lang.PersistentArrayMap
user=> (def a {:a 1})
#'user/a
user=> (class a)
clojure.lang.PersistentHashMap

after the patch:

user=> (class {:a 1})
clojure.lang.PersistentArrayMap
user=> (def a {:a 1})
#'user/a
user=> (class a)
clojure.lang.PersistentArrayMap

This should also lead to some minor performance enhancement since prior to this moment, every map def'ed would be a HashMap instead of an ArrayMap

So, I think patch #2 should be applied if not for this ticket's bug, at least for the reason stated above.
If somebody has any proposal for making this patch more solid regarding this ticket's bug, any help is welcome

This should not have passed screening. There are two issues, should be separate. I have no idea what has been screened nor what will be applied should it be approved. There's contention in the discussion but no resolution.

I don't think that there are two issues here.
The issue is only one: the compiler doesn't emit maps in a way consistent with what the reader returns and with how the compiler itself uses maps.
The symptoms are two: some interop calls fail, and def'ed vars with a literal map as value never use a PersistentArrayMap.

The underlying cause of those two symtoms is fixed by the patch 002 that i submitted (incorporated in Stu's clj944-plus-tests patch.

Stuart said that this approach feels fragile but the bug is caused by the fact that everywhere else clojure returns a PersistentArrayMap when the element count is <= than the PersistentHashMap threshold, and when emitting maps, it doesn't.

Making clojure emit maps consistently with how clojure does internally everywhere else looks to me like the only solution, and I don't really see how making clojure consistent is a fragile approach.

But again, if somebody can suggest a better solution to this problem, I'll gladly submit another patch.

Notes from the dev mailing list:

The "sources" JAR is generated by another Maven plugin, configured here:
https://github.com/clojure/clojure/blob/clojure-1.5.0-RC15/pom.xml#L169-L181

The simplest solution might be to just exclude the file from the sources jar. It looks like maven-source-plugin has an excludes option which would do the trick:

http://maven.apache.org/plugins/maven-source-plugin/jar-mojo.html#excludes