checkJs: x.js should use x.d.ts for type inference

Suggestion

🔍 Search Terms

checkjs, declaration files

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

If a declaration file with the same name than a JavaScript file is available (in the same location), then the declaration file should be used to check the js file.

📃 Motivating Example

Assume that "checkJs" and "strict" options are enabled.

Given the following files:

// @filename a.d.ts
export function f(n: number): number;

// @filename a.js
function f(a) {
    // Parameter 'a' implicitly has an 'any' type
    return a + 1
}

the type of f should be inferred and should thus type-check. TSC does not currently use a.d.ts to infer types of a.js.
Note that importing f in another js file, already infer f's type

// @filename b.js
import { f } from "./a.js"
// type of f is inferred

Start with the checkJs and strict reproducer using a.d.ts, a.js, and b.js, then run TypeScript's checker against it to confirm the current diagnostic. Trace how the declaration file is resolved for the imported f and compare that with checking a.js directly. Done means the same-name declaration is used for a.js type checking without changing emitted JavaScript.