Generic inference seems broken when optional chaining is involved

[michalburger1]

TypeScript Version: 3.7.2

Search Terms: generic inference optional chaining

Code

interface Base {
    doBaseStuff(): void
}

interface Derived extends Base {
    doDerivedStuff(): void
}

function a<T extends Base>(arg: T | undefined): T {
    if (arg === undefined) {
        throw new Error('It was undefined')
    }
    return arg
}

function f(arg?: { foo: Derived }): Derived {
    return a(arg?.foo)
    // Note that the following is completely okay:
    // return a(arg?.foo as Derived | undefined)
}

Expected behavior: T is inferred to be Derived and type checking passes

Actual behavior: T is inferred to be Base and type checking fails

Playground Link: http://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgEJwM4oN4ChkHIAmA9ulgMpgCuMMAFAJQBcyAbicEbgL666hIsRCgAi0YGwhFkEAB6QQRDGkw58hUuKiTpVWgxbtO3PrhjUQCMMBIhkcADwAVWQohKV5CAD56cKABzVlcAH2RLIggYUGkjVzxCZGAYZH8g5ABebIilaNiiRmREpMIwAAsoEgB3ZBAIWoBRKCqoegBySPz6onbGDQI+JKgIGih7AMDefgsrGztkBkmAflZsRZISVm1dGR4jHakZEoIRsYn0wOWAOhhN-p4gA

Related Issues:

[RyanCavanaugh]

Duplicate #34579; PR for 3.7.3 #34988