Instead of just mutex members we also consider mutex globals.
Unsurprisingly they are always in scope. Now the paper  says that
The scope of a class member is assumed to be its enclosing class,
while the scope of a global variable is the translation unit in
which it is defined.
But I don't think we should limit this to TUs where a definition is
available - a declaration is enough to acquire the mutex, and if a mutex
is really limited in scope to a translation unit, it should probably be
only declared there.