fix: signal setter callback form incorrect prev parameter type

[otonashixav]

Previously the prev parameter would erroneously be the same as the return type of the callback, instead of any type the signal could hold; see TS playground.

[coveralls]

Pull Request Test Coverage Report for Build 1320346031

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.908%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/solid/src/reactive/signal.ts	1	2	50.0%

Totals
Change from base Build 1319595723:	0.0%
Covered Lines:	1108
Relevant Lines:	1178

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💛 - Coveralls

[rturnq]

I think the whole setter function should also return T rather than U for the same reason as prev. As it is the return value will be narrowed to the new value only. Consider the following:

const [signal, setSignal] = createSignal(false) // boolean
let value = setSignal(true) // typeof value has been narrowed to true

value = false // Error: Type 'false' is not assignable to type 'true'

[otonashixav]

This test wants the setter to return U though. I also disagree that it should return T for the same reason as prev should be type T; the type of prev can't be known by the callback whereas the setter will always return U when passed U or a callback that returns U.

  test("Set signal returns argument", () => {
    const [_, setValue] = createSignal<number>();
    const res1: undefined = setValue(undefined);
    expect(res1).toBe(undefined);
    const res2: number = setValue(12);
    expect(res2).toBe(12);
    const res3 = setValue(Math.random() >= 0 ? 12 : undefined);
    expect(res3).toBe(12);
    const res4 = setValue();
    expect(res4).toBe(undefined);
  });

[ryansolid]

Sounds good.. want to move on the release tonight so going to merge and make the other change.