Breaking change to extends any causes confusing behavior with indirect references to any #38555

TypeScript Version: 3.9.2

Search Terms: #29571, "extends any"

Code

import ksm from "koa-session-minimal";

type KSMConfig = NonNullable<Parameters<typeof ksm>[0]>;
function initializeUserSession<T extends KSMConfig["store"]>(store: T): T {
    store.user = "test";
    return store;
}

Expected behavior:

store.user = "test" should succeed without failure.

Actual behavior:

Because KSMConfig["store"] is typed as any, due to the breaking change made in 3.9 to the behavior of extends any, the type gets reinterpreted as unknown and the compiler exclaims:

Property 'user' does not exist on type 'T'.(2339)

Playground Link: playground

Third-party typings for Javascript libraries often substitute any for incomplete types. This breaking change makes it hard to write functions that transform third-party types that reduce to any.

Using type assertions to fixup the type afterward is cumbersome.

    (store as KSMConfig["store"]).user = "test";

Could the change be relaxed to alias any to unknown only when any is explicitly requested? i.e.

function foo<T extends any>(x: T) {
    // x is unknown
}

type SomeType = any;
function bar<T extends SomeType>(x: T) {
    // x is SomeType (any)
}