Default parameter not accepted for conditional type although assignable to both branches

[jeysal]

TypeScript Version: Nightly playground 2020-07-09

Search Terms: conditional type not assignable, conditional branch assignable, default parameter assignable
(there are so many issues related to conditional types it's hard to tell which title may be related)

Code

Close to my use case:

type Options<T> = {
    t: T,
    flag?: T extends string ? boolean : false,
}

function f<Y>({ t, flag = false }: Options<Y>) { }

As minimal as I can get it, probably still the same cause:

type Param<T> = T extends string ? boolean : boolean;
function fn<T>(param: Param<T> = false) { }

Expected behavior:

No type error (other than unused parameters).

Note: This could be two separate bugs.
Example 1: The default parameter should be inferred to false, not boolean, like it is in const x = false, so if anything the error message should be Type 'false' is not assignable to [...]. It also does not seem to help to default to [...] = (false as false) or [...] = (false as const)
Example 2: Even with [...] ? boolean : boolean TS incorrectly does not allow to default to a boolean or false.

Actual behavior:

Example 1:

Type 'boolean' is not assignable to type 'Y extends string ? boolean : false'.(2322)

Example 2:

Type 'boolean' is not assignable to type 'Param<T>'.(2322)

Playground Link:
Example 1:
Playground Link
Example 2:
Playground Link

Related Issues:

[RyanCavanaugh]

Fix is in progress at #30639. It's a bit of a sticky wicket.

[jeysal]

Cool, looks like #26933 which that PR aims to fix (among other issues) is sufficiently similar to assume that this issue will be fixed by that as well. Closing as duplicate 👍