Contextually type class property initializers by extends/implements types #3667
Description
When a property has exactly one type from an extends or implements clause, we should contextually type that property's initializer by the type from the clause. For example:
interface ThingListener {
handleEvent: (x: MouseEvent) => void;
}
class Foo implements ThingListener {
handleEvent = x => {
// No error, but this code is incorrect
console.log(x.timestamp);
}
}Ideally we could take this one step further when widening. The following behavior is also undesirable:
interface HasLength {
length: number
}
class Foo implements HasLength {
// length: any, not really what we intended
length = undefined;
}
var x = new Foo();
x.length = 'wat'; // should not be allowede.g. #3666