Infer constrained generic parameters after instanceof check

TypeScript Version: 2.4.0

When a variable passes an instanceof check generic type with constraints on the type parameter, apply to constraints to the variable. There is currently no clean way to handle the following without an explicit cast:

Code

interface Foo {
    foo: string;
}

class Bar<T extends Foo> {
    constructor(readonly bar: T) {}
}

let a: any;
if (a instanceof Bar) {
    a.bar; // <-- a.bar should be 'Foo' instead of 'any'
}

Expected behavior:

a.bar is inferred as an instance of Foo.

Actual behavior:

a.bar is any

This change would only impact type checking, and will not affect compilation output.

Start with the TypeScript reproduction in the issue and inspect how the instanceof check narrows a variable whose generic type parameter has a constraint. Verify the inferred type of a.bar and identify the relevant type-checking test area. Done means a.bar is inferred as Foo without changing compilation output.