Preserve type aliases on implement interface

From https://github.com/microsoft/vscode/issues/90061

TypeScript Version: 3.8.1-rc

Search Terms:

• quick fix
• implement interface

Repo

export type UrlString = string;
export type UUID = string;

export interface I_IdToUrlMapper {
	mapIdToUrl(id: UUID): UrlString
	mapUrlToId(url: UrlString): UUID
}

export class IdToUrlMapper implements I_IdToUrlMapper {

}

Run implement interface on IdToUrlMapper

Expected behavior:
Type aliases are preserved

export class IdToUrlMapper implements I_IdToUrlMapper {
    mapIdToUrl(id: UUID): UrlString {
        throw new Error("Method not implemented.");
    }
    mapUrlToId(url: UrlString): UUID {
        throw new Error("Method not implemented.");
    }
}

Actual behavior:
Both methods use string:

export class IdToUrlMapper implements I_IdToUrlMapper {
    mapIdToUrl(id: string): string {
        throw new Error("Method not implemented.");
    }
    mapUrlToId(url: string): string {
        throw new Error("Method not implemented.");
    }
}

Related Issues

• https://github.com/microsoft/TypeScript/issues/34778

Start with the TypeScript language-service “implement interface” quick fix using the reproduced IdToUrlMapper example. Compare the generated method signatures with the expected output, and verify that type aliases such as UUID and UrlString are preserved rather than reduced to string.