Inferred index types on type parameters don't propagate through NonNullable<T>

[SLaks]

TypeScript Version: 3.0.1-insiders.20180726

Search Terms: I don't even

Code

class Endpoint<T = {}> { // This actually has 2+ parameters
    t!: T;
}

class ConfigReader<TEndpoint extends Endpoint> {
    constructor(readonly endpoint: TEndpoint) { }

    getValue(): NonNullable<TEndpoint['t']> {
        return this.endpoint.t;
    }
}

Expected behavior:

No compiler error

Actual behavior:

Type '{}' is not assignable to type 'NonNullable<TEndpoint["t"]>'.

Playground Link:
https://www.typescriptlang.org/play/index.html#src=class%20Endpoint%3CT%20%3D%20%7B%7D%3E%20%7B%20%2F%2F%20This%20actually%20has%202%2B%20parameters%0D%0A%20%20%20%20t!%3A%20T%3B%0D%0A%7D%0D%0A%0D%0Aclass%20ConfigReader%3CTEndpoint%20extends%20Endpoint%3E%20%7B%0D%0A%20%20%20%20constructor(readonly%20endpoint%3A%20TEndpoint)%20%7B%20%7D%0D%0A%0D%0A%20%20%20%20getValue()%3A%20NonNullable%3CTEndpoint%5B't'%5D%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20return%20this.endpoint.t%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A

Motivation:

In my actual code, ConfigReader has 4 type parameters, each of which is a further config class with even more nested parameters. I'm trying to avoid putting ~15 type parameters on ConfigReader by bundling them in this fashion, accepting a single type parameter for each top-level thing and accessing their nested type parameters via [] notation.

This works perfectly (so far), except for this issue.

[RyanCavanaugh]

We currently don't have a way to validate that the control flow of any particular function satisfies a conditional type (which NonNullable is) - a type assertion is needed here to indicate that you've correctly handled the case where this.endpoint.t is null/undefined