The new EventListener definition is not backwards compatible and/or has an error

[weswigham]

TypeScript Version: 2.7.0-dev.201xxxxx

I think was broken by #21889, and is not one of the changes listed as a fixed bug in the PR, so may be unintentional. The change has consequently has broken a handful of our RWC tests. Specifically, very common patterns like

Code

this._pseudoStatesToggle.addEventListener("DOMAttrModified", (mutationEvent: MutationEvent): boolean => {
                                                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2345: Argument of type '(mutationEvent: MutationEvent) => boolean' is not assignable to parameter of type 'EventListener'.
!!! error TS2345:   Type 'boolean' is not assignable to type 'void | { handleEvent(evt: Event): void; }'.    
                    return true;
                });

The change is that the webworker lib file now defines EventListener as:

type EventListener = (evt: Event) => void | { handleEvent(evt: Event): void; };

but it was:

interface EventListener {
    (evt: Event): void;
}

There also used to be a type:

declare type EventListenerOrEventListenerObject = EventListener | EventListenerObject;

that some RWC tests rely on (and this type looks to be what the alias is emulating).

[falsandtru]

microsoft/TypeScript-DOM-lib-generator#352 breaks the acceptability (type safety) of interfaces depended on EventListener type. This is a large breaking change. This change should be reverted.

[weswigham]

EventListenerOrEventListenerObject is also relied upon by some real code bases (even if it should just be an alias for EventListener now).
Also,

type EventListener = (evt: Event) => void | { handleEvent(evt: Event): void; };

is probably just straight wrong, even by original intent, since that is parsed as "A function with an argument of type Event and a return type of either (void OR an object with a handleEvent method)" and not "(A function that returns void with an argument of type Event) OR (an object with a handleEvent method)."