--esModuleInterop: Strange incompatibility between correct and incorrect import of same type

[ghost]

TypeScript Version: master

Code

b.d.ts

declare class C {
    f: this;
}
declare namespace C {}
export = C;

a.ts

import * as C1 from "./b";
import C2 = require("./b");

function f(t: (c: C1) => void): void {
    t(new C2());
}

Expected behavior:

Error at import * as C1.

Actual behavior:

No error at the import declaration. Error at new C2():

src/a.ts(5,7): error TS2345: Argument of type 'C' is not assignable to parameter of type 'C'.
  Types of property 'f' are incompatible.
    Type 'C' is not assignable to type 'this'.

(possibly related: #20704)

[weswigham]

This is because the redundant better-error-check in checkTypeRelatedTo doesn't apply to contravariant failures right now. See ~lines 9384. It only checks if the source had an originating import and an unmangled type - to handle contravariant positions, the target should be checked as well. Not related to #20704, I don't think.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[ghost]

@weswigham It sounded like there was a potential fix for this? Should we keep this issue around?

[weswigham]

There's definitely a potential fix. The current implementation just doesn't handle checking for a better error for contravariant comparisons.