Narrowing breaking changes

[mhegazy]

This is a place holder for changes to the type narrowing behavior in TypeScript 2.0:

• Generic type parameters are now narrowed

function g<T>(obj: T) {
    var t: T;
    if (obj instanceof RegExp) {
         t = obj; // RegExp is not assignable to T
    }
}

See the explanation by @Arnavion in #7719 (comment)

• Type narrowing does not work for catprued variables in functions and class expressions

export class Class {
    constructor(public p) { }
}

var c: Class | string;
var x: Class;

if (c instanceof Class) {
    function inner() {
        x = c; // Error, type of c is not narrowed, c is Class | string
    }
    x = c; // OK, c is Class
}

[ivogabe]

In the first example obj should be narrowed to T & RegExp, that seems more correct. Is that doable?

[mhegazy]

narrowing makes the type more "specific", this seems like expanding the domain of the type.

[ivogabe]

I thought that "more specific than" means "subtype of". That would mean that narrowed types are always a subtype of their original type and also always assignable to the original type. RegExp is not a subtype of all possible types for T, since it is a type argument, but T & RegExp is a subtype of it. Or is there something that I missed?

[mhegazy]

Type parameter narrowing is tracked by #8563