Create subtype reduction rules for assertion functions

Bug Report

🔎 Search Terms

inference assert

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs

⏯ Playground Link

Playground link with relevant code

💻 Code

function Test<X extends any>(params: X[]) {}
function Test2<InParams extends readonly any[]>(params: InParams) {}

function f1(a: any): number {return 1}
function f2(a: any): string {return "string"}
function f3(a: any) {return {k: 1}}

function a1(a: any): asserts a is number {}
function a2(a: any): asserts a is string {}
function a3(a: any): asserts a is {k: number} {}

const aifwe = [f1, f2, f3]
Test([f1, f2, f3])
Test([a1, a2, a3])
Test2([f1, f2, f3])
Test2([a1, a2, a3])

🙁 Actual behavior

The Test and Test2 calls are inferred as (in order):

• function Test<((a: any) => number) | ((a: any) => string) | ((a: any) => { k: number; })>(params: (((a: any) => number) | ((a: any) => string) | ((a: any) => { k: number; }))[]): void
• function Test<(a: any) => asserts a is number>(params: ((a: any) => asserts a is number)[]): void
• function Test2<(((a: any) => number) | ((a: any) => string) | ((a: any) => { k: number; }))[]>(params: (((a: any) => number) | ((a: any) => string) | ((a: any) => { k: number; }))[]): void
• function Test2<((a: any) => asserts a is number)[]>(params: ((a: any) => asserts a is number)[]): void

🙂 Expected behavior

For the assert calls to also be inferred as unions.

This is also irregular since with the wrong inference, I would expect the other functions to be marked as errors, since they have different type signatures.

Start with the linked TypeScript Playground example and trace generic inference for arrays of assertion functions, focusing on the requested subtype reduction rules. Done when the Test and Test2 assertion calls infer unions like the ordinary function calls, with the relevant inference behavior covered by a regression test.