Flag for strict default function `this` types for call-site and assignability checking (`--strictThis`)

[mattmccutchen]

#6018 was originally going to add a --strictThis flag that made function this types default to void or the enclosing class (instead of any) for purposes of call-site and assignability checking, but that functionality was dropped (details). This is a follow-up suggestion for the dropped functionality, whatever the flag ends up being named.

[mhegazy]

Please see #7689 for the relevant desing discussion

[jcalz]

I just ran into a problem where the obvious solution would be that methods without an explicit this parameter default it to the this type. Searching brought me here. So the main problem is... performance? In any case, I hope a --strictThis option makes it into TS sometime (adding my 👍 ).

[flash1293]

Maybe we can revive this thing as I believe it would bring a lot of benefit.

Regarding the "backwards compatibility" problem - is this due to class-this context for methods which in reality should stay any because they are allowed to be called from different contexts? If yes, --strictThis could only implicitly set the context to class-this for methods which have a typescript implementation and actually use this in the method body. DefinitelyTyped could adapt step by step and pure typescript code bases could use the improved type check right away.

[tejasmanohar]

I also hope this is added back. I just started using Typescript last week, and this is basically the only "type error" I've run into (and repeatedly for that matter) in my all Typescript codebase. I hit it frequently, as I am using an ES6 classes as a router/controller in my Express app, e.g. app.use(this.someClassFunction). When I see failing tests with an error like cannot access property someClassFunction of undefined, I just change the declaration syntax to ES7 function bind syntax class X { constructor() {} x: (req, res) => {} } and the issue is fixed, but it would be really nice to know about that at compile time instead of at runtime.

I've been impressed with Typescript in general but was very surprised to hear that it's not checked at compile time since I can't think of another typed language, where such an error is possible.