<p></p>
<p><b>@maribu</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/16465#discussion_r647276462">sys/Makefile.dep</a>:</p>
<pre style='color:#555'>> @@ -698,6 +698,7 @@ ifneq (,$(filter xtimer,$(USEMODULE)))
     endif
   else
   # ztimer_xtimer_compat is used, all of *xtimer's API will be mapped on ztimer.*
+  NO_AUTOLOCATE += xtimer
</pre>
<p>The location of modules is done fully again at each recursion step. If xtimer would be auto-located in a transient state, it would be dropped later on. But some care should be taken so that an <code>Makefile.dep</code> doesn't do anything <code>xtimer</code> related when <code>ztimer_xtimer_compat</code> is also used. But since there is no <code>Makefile.dep</code> in <code>sys/xtimer</code> and in <code>sys/Makefile.dep</code> (above code) the correct guards are in place, this should just work.</p>
<p>A bit of a foot gun, but choosing different implementation options for the same API is just not properly modeled in the build system, so some friction is expected.</p>

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