I haven't tried with a five year old version of SBCL, but even early on (from the beginning of the project?) SBCL treated type declarations (using DECLARE) as a

I haven't tried with a five year old version of SBCL, but even early on (from the beginning of the project?) SBCL treated type declarations (using DECLARE) as assertions (as opposed to mere hints for optimization), making it what some might call a gradual-typed system. So with properly declared type, SBCL could have found it then too. Now, some discourage the use of type declarations in Common Lisp (for a variety of reasons, one being the limited portability as less clever compilers don't warn against mismatch, but might produce broken code) and suggest the (run-time type testing) CHECK-TYPE (standard) macro instead. SBCL's type inference (at least in recent versions) is sufficiently clever to detect mismatches at compile time, when the type is (implicitly) announced using CHECK-TYPE.