Description:

Uncomments the tests that were skipped when enabling type checking for the test suite. And adds overload signatures with type guards to first and last.

Note that because the predicates are optional, additional overload signatures that match calls made with no arguments or with a null or undefined predicate are required. Otherwise, the S type parameter is inferred as {} and the type of the output observable becomes Observable<{}>.

Related issue (if exists): #3717