PP0005

might not be a function

The expression is an unknown value that might not be a function, or might be a function that has side effects on the heap or that could throw an exception.

If this should happen to the case, then the values of the heap computed by Prepack will be incomplete or wrong, with the result that running Prepack over a program changes its runtime behavior.

To make this error go away, consider refining the environment model so that Prepack can figure out that this expression evaluates to a unknown function that is safe to call.