When the namespace component of an auto-resolving keyword literal refers to an undeclared alias, the reader produces an error, like so:
For symbols, however, no such syntax exists. This is a common source of errors with functions like clojure.spec.test.alpha/instrument which accept qualified symbols of var names as arguments. Currently, the go-to notation for such symbol literals is syntax quote which of course accepts any namespace. Now, when one accidentally refers to an alias in one of those symbolic arguments which hasn't been declared in the current namespace (e.g. when transplanting some code from one namespace to another), it would be very useful to have the same error behavior as with auto-resolving keywords. To stick with the instrument example, one would end up calling potentially expensive real functions instead of the mocks which could easily go unnoticed.
This is a bit tricky. The first thing that comes to mind is double backtick, as in ``foo/bar, but that already has meaning and is likely to already be used in existing macros. I guess %foo/bar could work, though there is of course no guarantee that this isn't already in use in actual namespace names, too.
As you can tell, I haven't done an extensive survey of possible syntax options, yet, nor have I started working on a patch. Just wanted to solicit opinions on the issue to see if it's worth pursuing any further.