Suboptimal inference to unions like T | Promise<T>

[ahejlsberg]

TypeScript Version: 3.6.0-dev

Code

declare function foo<T>(x: T | Promise<T>): void;
declare let x: false | Promise<true>;
foo(x); // Error, 'false | Promise<true>' not assignable to 'true | Promise<true>'

declare function bar<T>(x: T, y: string | T): T;
const y = bar(1, 2); // Error, '2' not assignable to 'string | 1'

Expected behavior: No errors.

Actual behavior: Errors as noted above.

Playground Link: https://www.typescriptlang.org/play/index.html#code/CYUwxgNghgTiAEAzArgOzAFwJYHtVJxwB4AVAPgAoAPALnhPgB94AFGHAWywGcRSyAlHQBuOLMADcAWABQoSLAQQQGeLSRQIvJq3ZdeRDDGQgy0mYkLUBE+PAD09+AFEY7GLNnzocJGky4+ABGsPzUdCQANPAAnnTcRlioAOY6JEL05mB4CbHwALzwITAUAIzRAEw2do4ubjgeMkA

Related Issues: Discovered in #32386.