When doing a type conversion from bool to interface{} as part of a return statement, the code does not behave as expected.

See: http://www.gopherjs.org/playground/#/GM1vWfnqn_

Note that if you change the runPredicate to store the values in an intermediate before passing it on, it behaves as expected. Also note that if the predicate test method is single-value rather than multi-value, it works as well.