Better forward typings (TS)

[bloadvenro]

Fixed wrong forward typings behavior, when forwarding FROM "less strict" type TO "more strict" type throws no errors.

[vercel]

This pull request is being automatically deployed with ZEIT Now (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://zeit.co/zerobias/effector/iusx0eiga
🌍 Preview: https://effector-git-fork-bloadvenro-forward-typings.zerobias.now.sh

[bloadvenro]

Also added tests for providing both generics (as you asked me in our community) to avoid accidental breaking change in future.

[bloadvenro]

@abliarsar proposed an idea how to make forward inference even better :)

  it('should forward from `Unit<*>` to `Unit<void>`', () => {
    const from = createEvent<string>()
    const to = createEvent<void>()

    forward({from, to}) // <-- NOW FIXED
...

[bloadvenro]

Remark:

const str = createEvent<string>()
const strOrNum = createEvent<string | number>()

// TS v3.5.3 (current version in effector repo)
//
// Produces an error:
//
// Type 'Event<string | number>' is not assignable to type 'Unit<string>'.
//     ...
forward({
  from: strOrNum,
  to: str
})

// TS v3.6.3 (current stable TS version)
//
// Produces an error:
//
// No overload matches this call.
//     ...
forward({
  from: strOrNum,
  to: str
})

This is just a note.

[zerobias]

Great! 👍