Iv'e read alot of the Typescript github issues regarding implementation but there's little documentation or people working in this area.
My goal is to basically have typescript understand that..
[1,2,3,4,5].filter(_ % 2).map(_ + 2)
[{name: "test"}, {name: "test2"}].map(_.name).map(_.toUpperCase())
is the same as...
[1,2,3,4,5].filter((x) => x % 2).map((x) => x + 2)
[{name: "test"}, {name: "test2"}].map((x) => x.name).map((x) => x.toUpperCase())
i either want to define a SweetJS macro to convert all underscores to (x) => x at compile time or (peferably) have typescript apply that transformation for me since SweetJS is fairly hacky.
(For reference https://github.com/sweet-js/sweet-core)
How would i go about implementing something that allows typescript to at the bare minimum understand underscore syntax in the correct context \AND\ if possible transform underscores at compile time.
EDIT3: (Edit3 is here for readability) this is how far i have got with the problem, i have opted not to change the compiler to think it is a different type but instead do this....
declare const _: (<T>(arg: T) => T) & { [key: string]: () => any }
const test = [{name: "Shanon", age: 24}].filter(_.age);
which typescript is now happy with, but it is not typesafe so any help would be appreciated.
EDIT: With my minimal compiler skills, i think that i need typescript to understand that when i am within the context of a "CallExpression" underscore has a very unique meaning (CallExpression in the AST).
EDIT2: 
this is the AST defintiion i need outputted (something like this) it's important to note, i don't want the underscore converted before it is typechecked but i want to controle how a underscore is type checked. It should only treat the underscore as this type inside a CallExpression AST type