Type assertions change type inference

[bowenni]

TypeScript Version: It is reproducible in 3.4.5 and 3.5.0-dev.20190515 but works in 3.3.3333.

Search Terms:
type assertion, type inference

Code

declare const foo: Object;
declare function f<V = any, R = any>(p: {[key: string]: V}): V|R;
f(foo);            // mouse over f, its type is f<any,any>
f(foo) as string;  // mouse over f, its type is f<string,string>

Expected behavior:
No errors.

Actual behavior:

Argument of type 'Object' is not assignable to parameter of type '{ [key: string]: string; }'.
 f(foo) as string;  // mouse over f, its type is f<string,string>
   ~~~

It seems like the as string type assertion changes the type inference of V and R, which is quite surprising. Is this a regression or intended?

Playground Link:
https://www.typescriptlang.org/play/#src=declare%20const%20foo%3A%20Object%3B%0D%0Adeclare%20function%20f%3CV%20%3D%20any%2C%20R%20%3D%20any%3E(p%3A%20%7B%5Bkey%3A%20string%5D%3A%20V%7D)%3A%20V%7CR%3B%0D%0Af(foo)%3B%20%20%20%20%20%20%20%20%20%20%20%20%2F%2F%20f%20is%20f%3Cany%2Cany%3E%0D%0Af(foo)%20as%20string%3B%20%20%2F%2F%20f%20is%20f%3Cstring%2Cstring%3E

Related Issues:
I found #28816 (comment) which gives me a hint that TypeScript might determine the type of a type parameter in some order of preference too, similar to a parameter.
My guess is that the as string type assertion provides the contextual generic type, which is before the initializer type any but what do I know.

[ahejlsberg]

This is working as intended. The type assertion establishes a contextual type and we infer from that type to the return type of the function.