Generic type inference does not appear to fill in type of this parameter

[ghost]

TypeScript Version: nightly

Code

function f<T>(x: T, f: (x: T) => string): void {}
f("", x => x);

function g<T>(x: T, f: (this: T) => string): void {}
g<string>("", function() { return this; });
g("", function() { return this; });

Expected behavior:

No error. this is always of type string.

Actual behavior:

In the last line, the type of this is still T; it hasn't been 'filled in'.

src/a.ts(6,7): error TS2345: Argument of type '() => T' is not assignable to parameter of type '(this: string) => string'.
  Type 'T' is not assignable to type 'string'.

[sandersn]

Duplicate of #9673 with a proposed fix at #9746.