Add `getUnknownType` to `checker` api

[kirkwaiblinger]

🔍 Search Terms

unknown, ts api, type relationship api,

✅ 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I would like to add a method to the checker api to get an intrinsic unknown type.

    const checker: TypeChecker = {
         // ...
         getAnyType: () => anyType,
+        getUnknownType: () => unknownType,
         getStringType: () => stringType,
        // ...
    }

📃 Motivating Example

I want to write a function that looks roughly like

import * as ts from 'typescript';
import * as tsutils from 'ts-api-utils';

export function getConstrainedType(checker: ts.TypeChecker, type: ts.Type): ts.Type {
  return !tsutils.isTypeParameter(type)
    ? type
    : checker.getBaseConstraintOfType(type) ?? checker.getUnknownType();
}

for use in typescript-eslint (see the utility function getConstrainedTypeAtLocation, which currently cannot be used to resolve typescript-eslint/typescript-eslint#10314 (comment), due to an unconstrained generic not returning an unknown type with that utility's current implementation)

I'm happy to submit a PR to make this change.

💻 Use Cases

1. What do you want to use this for?
   • fix(eslint-plugin): [await-thenable, return-await] don't flag awaiting unconstrained type parameter as unnecessary typescript-eslint/typescript-eslint#10314 (comment)
2. What shortcomings exist with current approaches?
   • cannot write linter code that handles the types irrespective of whether it is a generic or ordinary type, due to no way to map an unconstrained generic to an intrinsic unknown type object
3. What workarounds are you using in the meantime?
   • separate code paths 🙁, see fix(eslint-plugin): [await-thenable, return-await] don't flag awaiting unconstrained type parameter as unnecessary typescript-eslint/typescript-eslint#10314 (comment)