thisType of callback parameter isn't enforced if the callback has another parameter

[Arnavion]

TypeScript Version: 2.1.0-dev.20160910

Code

declare function foo(cb: (this: number, arg: string) => void): void;

foo(function (): void { this.length; }); // (1)
foo(function (arg: string): void { this.length; }); // (2)

tsc -t es5 ./foo.ts

Expected behavior:

Both (1) and (2) should complain about length not existing on number

Actual behavior:
(1) correctly complains: foo.ts(3,30): error TS2339: Property 'length' does not exist on type 'number'. But there is no error for (2).

Adding --noImplicitAny doesn't change the result.

Adding --noImplicitThis gives foo.ts(4,36): error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation. which confirms that this in (2) is getting inferred as any

[RyanCavanaugh]

Same root cause as #4241 - due to the arg: string type annotation, the entire function is not contextually typed. #4241 removes the restriction that type annotations in functions "turn off" contextual typing. Should be an easy change if you want to take a stab at it.

[Arnavion]

Ah, you're right. The : string annotation is unnecessary and removing it allows this to be inferred correctly. I didn't think to try that.