non-null assertion on TypeParameter changes `string | undefined` to `String`

[ajafff]

TypeScript Version: 2.7.0-dev.20180123

Search Terms:

Code

declare function foo(p: string): void;

function fn<T extends string | undefined, U extends string>(one: T,two: U) {
    let three = Boolean() ? one : two;
    foo(one!);
    foo(two!);
    foo(three!); // error on this line
}

Expected behavior:
No type error.

Actual behavior:

Argument of type 'string | String' is not assignable to parameter of type 'string'.
  Type 'String' is not assignable to type 'string'.
    'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.

This is caused by #20995. To be fair, without that change the above was not valid at all.

Playground Link:

Related Issues:
Maybe related to #21317

[ahejlsberg]

@weswigham

[weswigham]

getApparentTypeForLocation (which is how we enabled this, similarly to how we've made guards on this.foo.bar work when this.foo is an indexed access type) uses getAparrentType. That replaces primitives with their corresponding interfaces; I don't think we (ever) actually want do do that here (it just didn't matter when it was only ever used for the left hand side of a property access), so it's probably appropriate to factor out the constraint-mapping behaviors and perform only that here.