TS doesn't allow optional param destructing in spite of having a default value assigned

[jalooc]

TypeScript Version: 3.7.2

Search Terms:
default optional parameters destructing

Code

type F = {
  (foo?: { bar?: {}}): void
}

const f: F = ({ bar } = {}) => {

}

const g: F = (foo = {}) => {
  const { bar } = foo
}

Expected behavior:
Both functions: f and g should be compiled without errors. foo is optional, but it's also provided a default value; same with bar. Therefore both foo and bar are in fact never undefined. Btw this also works in vanilla js, tested with all sorts of parameter types.

Actual behavior:
TS error is thrown for the f function's destructuring of the optional param:

Property 'bar' does not exist on type '{ bar?: {} | undefined; } | undefined'.(2339)

This error does sound right, but shouldn't occur taking into consideration those three facts:

1. If foo is undefined (= not passed as an argument to the function), the default value kicks in, so foo becomes {}. It's safe to access bar then.
2. Function g does exactly the same logic as f, yet it doesn't fail compilation.
3. It's safe to access bar in vanilla js.

Playground Link:
http://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=11&pc=2#code/C4TwDgpgBAYlC8UDeAoKUAUAzA9jg-AFzJQBGAhgE5HIC+tAlMQG44CWAJirSigMY4AdgGdgULMTiIMSMlSi0EdBggB8yXj35DRUAOaSl2PEqSM1G9AJFjZFSgqW4c3IA

[RyanCavanaugh]

TypeScript is doing what you told it to do: you wrote bar without any annotation, so it was assumed to be a required destructuring.

You can write this instead if you meant it to be optional:

const f: F = ({ bar }: { bar?: {} | undefined } = {}) => {

}

[jalooc]

@RyanCavanaugh bar is typed withing the F type. Isn't that enough?

Besides ? implies it can be undefined. Isn't adding undefined type to the union for an optional param a no-op?

[RyanCavanaugh]

Contextual typing can change the type of a parameter, but can't change its optionality. | undefined is implied by optionality, but not vice versa.

[jalooc]

@RyanCavanaugh Exactly, so adding | undefined to bar in my example doesn't change anything since bar is optional. I don't see how this is relevant to my issue anyway. What do you mean by:

you wrote bar without any annotation

? It's type is derived from F.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.

[jalooc]

@typescript-bot It's still not clear to me whether this really is a question or a bug, as my last comment hasn't been answered. Please don't close until there's a unanimous conclusion about it.