Comment 1 for bug 365497

CLHS says about LOOP COUNT:

"The count construct counts the number of times that the supplied form returns true. The argument var accumulates the number of occurrences; if var is supplied, loop does not return the final count automatically. The var argument is bound as if by the construct with to a zero of the appropriate type. Subsequent values (including any necessary coercions) are computed as if by the function 1+. If into var is used, a type can be supplied for var with the type-spec argument; the consequences are unspecified if a nonnumeric type is supplied. If there is no into variable, the optional type-spec argument applies to the internal variable that is keeping the count. The default type is implementation-dependent; but it must be a supertype of type fixnum."

So FIXNUM appears to be a legal assumption, actually. Suboptimal, but legal.