Lack of type checking for callback parameters

[falsandtru]

TypeScript Version: master

Code

function f(g: (ev: Event) => any) { }
function g(ev: MouseEvent) { }
f(g);
f(ev => g(ev));

Expected behavior:

$ node built/local/tsc.js index.ts --noEmit --strict
index.ts(3,3): error ...
index.ts(4,11): error TS2345: Argument of type 'Event' is not assignable to parameter of type 'MouseEvent'.
  Property 'altKey' is missing in type 'Event'.

Actual behavior:

$ node built/local/tsc.js index.ts --noEmit --strict
index.ts(4,11): error TS2345: Argument of type 'Event' is not assignable to parameter of type 'MouseEvent'.
  Property 'altKey' is missing in type 'Event'.

[mhegazy]

https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant

[falsandtru]

I didn't know that well because the FAQ doesn't say specific definitions of types of Animal and Dog. But it seems like you can keep the consistency of f(g) and f(ev => g(ev)). Are you really choosing the current inconsistency?

[RyanCavanaugh]

Suffice to say we've thought about this problem a lot and are sticking with the current situation.