Type checking doesn't work with extra function arguments

[falsandtru]

Extra argument types are inferred correctly but aren't checked. Looks like #17559 is implemented incompletely. And this code also breaks colorization by vscode like the following code.

function f(): <T extends string>(a: T) => void {
  return (a, b = a) => {
    isFinite(a);
    isFinite(b);
  }
}

cc @DanielRosenwasser @RyanCavanaugh

TypeScript Version: master

Search Terms:

Code

const f: () => void = (a = '') => isFinite(a); // should be error

Expected behavior:

Actual behavior:

Playground Link:

Related Issues: