Forward compatible type definitions regarding compiler warnings

## Search Terms forward compatibility, experimental features, forward compatible, undeclared types, new browser features, warning undeclared overloads ## Suggestion Currently if a TypeScript `.d.ts` file contains a function overload with an unknown type it doesn't discard the type but rather treats it like there was an overload that produces `any` as the return type. This is more easily demonstrated with an example: ```js // plus.js function plus(x, y) { return x + y } ``` ```ts // plus.d.ts export default function plus(a: number, b: number): number // Forward compatible definition that should be ignored by older // versions of TypeScript that don't recognise it or by // consumers using a "target"/"lib" that isn't "esnext" (or whatever version // bigint eventually ends up in) export default function plus(a: bigint, b: bigint): bigint export default function plus(a: string, b: string): string ``` Now if we use the function like so: ```js const value = plus({}, {}) ``` Will produce different results based on if the `--lib`/`--target` supports `bigint or not. In a world that does support BigInt we will get a warning: `Argument of type '{}' is not assignable to parameter of type 'string'` which means if people are in the future world then it behaves just fine. But if they use it in a non future world then no warning is issued and the type of `value` is inferred to be `any`. NOTE: The fact `bigint` is used here isn't really important it was just a simple example to write, this applies to any differences between needed target of the `d.ts` file and target of the consumer. ## Use Cases The main use case is for libraries that wish to be forward compatible with types that might not be configured in `--lib`/`--target` by feature detection (or by just working in the case of the example). ## Examples I'm not sure what would be the best way of supporting this, probably another flag like `--warnings-ignore-unrecognized-overloads` or something along those lines. Alternatively it…