This type parsing error and incorrect type inference

[stephanedr]

TypeScript Version: 2.0.0 (I suppose beta, though tsc -v just reports 2.0.0)

Code

function test(e: HTMLElement): void {
    e.onclick = (this: HTMLElement, event) => { }; // error TS1005: ')' expected.
                                                   // error TS1005: ';' expected.
                                                   // error TS1128: Declaration or statement expected.

    e.onclick = function(this: HTMLElement, event) { }; // error TS7006: Parameter 'event' implicitly has an 'any' type
}

Expected behavior:
1st: no parsing error.
2nd: event: MouseEvent inference, as for e.onclick = function(event) { };

Actual behavior:

[RyanCavanaugh]

This is still going to be an error because it doesn't make sense for an arrow function to specify its this parameter (it gets the outer this), but we should give a semantic error instead of a parse error

[RyanCavanaugh]

Regarding the second thing, this is because we don't contextually type parameters when any parameter has a type annotation. There's a bug on that somewhere but I can't find it at the moment; we should revisit it (if it's closed) at the very least for the case where the only annotated parameter is the this the parameter

Edit: #4241

[mattmccutchen]

Let me add on to this issue (it doesn't seem worth filing another): a this parameter on a generic arrow function parses but has no effect:

let f = <T>(this: number) => {  // no error
    let that = this;  // resolved the same way as if `this: number` were not specified;
                      // may be an error depending on context and compiler options
};

[mattmccutchen]

Fix is up at #25863.