Summary:
In Flow v0.99 we are changing function type annotations to be strict about their
static properties. This causes a small issue with the union of a Subcription and
`() => mixed` function type, where the latter is now understood to possibly have
an `unsubscribe` property with a mixed type.
This causes the following refinement check, which appears lower in this file, to
cause an error in the next version of Flow:
```
if (cleanup) {
if (cleanup.unsubscribe) {
cleanup.unsubscribe(); // <-- error here
}
// ...
}
```
In Flow v0.99, because `() => mixed` statics are now checked, Flow sees that
`cleanup.unsubscribe` might be `mixed`, which passes the conditional but could
cause an exception when called.
I also needed to change JestMockFn to have exact statics, because tests
sometimes use a value with that type in place of the cleanup function. That runs
into the `{} <: {+p?: void}` rule, which is an error. `{||} <: {+p?:void}` is
not an error.
Reviewed By: josephsavona
Differential Revision: D15522655
fbshipit-source-id: 2ae3c9016e2b07abaac79827082d2f8743623eb5
ab1a42762c