Infer optional types for IIFE parameters with missing arguments

[ahejlsberg]

We currently contextually type un-annotated IIFE parameters by their corresponding arguments. This PR expands upon this concept by inferring un-annotated IIFE parameter with missing arguments as optional. That allows us to handle the following common idiom with no errors:

let foo = (function (window, undefined) {
    // Body
})(window);

Without this PR the example is an error because the undefined isn't marked optional.

Note that the PR also simplifies some overly complicated logic for computing the minArgCount property in signatures.

[ahejlsberg]

@billti @mhegazy This is a fix for an issue Bill mentioned the other day.

[billti]

Great!

Just playing around with this in TypeScript, and with strictNullChecks set, the below gives an error with the type annotation on b. Without the annotation on b it is fine (and also obviously if I pass undefined for the second arg).

Maybe this is "by design", and if you put the annotation you need to mark the param as optional (which also gets rid of the error), but intuitively it feels like this should be valid with the new inference. Thanks!

(function(a: string, b: undefined) {

})("test");

[ahejlsberg]

@billti Yes, this is by design. We only contextually type un-annotated parameters. Once a type annotation is present you must also add a ? modifier if you want the parameter to be considered optional.

[mhegazy]

@sandersn this will need to be covered in your change for initialized parameters.