Added some comments.

src/util/CopyableSynchronization.h

@@ -10,6 +10,9 @@ namespace ad_utility {
 // a new mutex. This is sufficient for applications like in
 // `getInternallyVisibleVariableColumns()` where we just want to make a
 // `const` member function that modifies a `mutable` member threadsafe.
+// Note that a copy-constructed `CopyableMutex` will be unlocked, even if the
+// source was locked, and that copy assignment also never locks or unlocks any
+// of the involved atomics.
 struct CopyableMutex : std::mutex {
   using std::mutex::mutex;
   CopyableMutex(const CopyableMutex&) {}

test/CopyableSynchronizationTest.cpp

@@ -13,12 +13,15 @@ using namespace ad_utility;
 TEST(CopyableSynchronization, CopyableMutex) {
   // Not much to test here.
   CopyableMutex m1;
-  [[maybe_unused]] CopyableMutex m2{m1};
-  m1 = m2;
   m1.lock();
+  [[maybe_unused]] CopyableMutex m2{m1};
+  // m2 is still unlocked.
   EXPECT_TRUE(m2.try_lock());
-  m1.unlock();
   m2.unlock();
+  m1 = m2;
+  // m1 is still locked;
+  EXPECT_FALSE(m1.try_lock());
+  m1.unlock();
 }
 
 // _________________________________________________