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

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)

Reproduce the report using b.d.ts and a.ts exactly as shown, then run the TypeScript checker to observe the diagnostic at new C2(). Start by tracing how import * as C1, import-equals require, export =, and the this-typed property are compared. Done means the incompatibility is handled consistently, with the expected diagnostic location or behavior covered by a regression test.