Bug: Conditional types don't properly propagate this

[SLaks]

TypeScript Version: 3.1.1-insiders.20180926

Search Terms: this conditional

Code

class Builder<T1, T2, T3, TWith>{
    private _noStructuralTyping: undefined;

    withBroken<TAlsoWith>() {
        return this as AddWith<this, TAlsoWith>;
    }
    withBroken2<TAlsoWith>(): AddWith<this, TAlsoWith> {
        return this;
    }
    withWorking<TAlsoWith>(): Builder<T1, T2, T3, TWith|TAlsoWith> {
        return this;
    }
}


type AddWith<TBuilder, TAlsoWith> = TBuilder extends
    Builder<infer T1, infer T2, infer T3, infer TWith>?
    Builder<T1, T2, T3, TWith|TAlsoWith>:
    never;

const test = new Builder<1, 2, 3, string>().withBroken<number>();

// Without using the `this` type, it works as expected:
const simpleTest = new Builder<1, 2, 3, string>();
type WithNumber = AddWith<typeof simpleTest, number>;
const demo: WithNumber = simpleTest;

Expected behavior:

this should be convertible to AddWith<this, whatever>

Actual behavior:

Type 'this' is not assignable to type 'AddWith<this, TAlsoWith>'.
  Type 'Builder<T1, T2, T3, TWith>' is not assignable to type 'AddWith<this, TAlsoWith>'.

Motivation:
I'm building a builder class which builds configuration both in the type system and at runtime.

I don't want to repeat all of the existing type parameters (which have long names) in every method that adds a type to TWith

Playground Link: http://www.typescriptlang.org/play/#src=class%20Builder%3CT1%2C%20T2%2C%20T3%2C%20TWith%3E%7B%0D%0A%20%20%20%20private%20_noStructuralTyping%3A%20undefined%3B%0D%0A%0D%0A%20%20%20%20withBroken%3CTAlsoWith%3E()%20%7B%0D%0A%20%20%20%20%20%20%20%20return%20this%20as%20AddWith%3Cthis%2C%20TAlsoWith%3E%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20withBroken2%3CTAlsoWith%3E()%3A%20AddWith%3Cthis%2C%20TAlsoWith%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20return%20this%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20withWorking%3CTAlsoWith%3E()%3A%20Builder%3CT1%2C%20T2%2C%20T3%2C%20TWith%7CTAlsoWith%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20return%20this%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0A%0D%0Atype%20AddWith%3CTBuilder%2C%20TAlsoWith%3E%20%3D%20TBuilder%20extends%0D%0A%20%20%20%20Builder%3Cinfer%20T1%2C%20infer%20T2%2C%20infer%20T3%2C%20infer%20TWith%3E%3F%0D%0A%20%20%20%20Builder%3CT1%2C%20T2%2C%20T3%2C%20TWith%7CTAlsoWith%3E%3A%0D%0A%20%20%20%20never%3B%0D%0A%0D%0Aconst%20test%20%3D%20new%20Builder%3C1%2C%202%2C%203%2C%20string%3E().withBroken%3Cnumber%3E()%3B%0D%0A%0D%0A%2F%2F%20Without%20using%20the%20%60this%60%20type%2C%20it%20works%20as%20expected%3A%0D%0Aconst%20simpleTest%20%3D%20new%20Builder%3C1%2C%202%2C%203%2C%20string%3E()%3B%0D%0Atype%20WithNumber%20%3D%20AddWith%3Ctypeof%20simpleTest%2C%20number%3E%3B%0D%0Aconst%20demo%3A%20WithNumber%20%3D%20simpleTest%3B
Related Issues: #28159

[ahejlsberg]

This is working as intended. We generally don't reason about assignability of conditional types applied to generic types. For this reason, neither this nor AddWith<this, TAlsoWith> is considered assignable to the other, so the assertion fails. In order for the code to type check you'll need to assume responsibility for verifying the assertion by first converting to any:

return this as any as AddWith<this, TAlsoWith>;