Readonly optional properties break type-checking

[JSMonk]

Bug Report

🕗 Version & Regression Information

Found in TS v4.8.2.
This is the behavior in every version I tried, and I reviewed the FAQ for entries about

⏯ Playground Link

Playground link with relevant code

💻 Code

// Original Case
declare function foo(obj: { readonly constructor?: any }): void

foo(Object) // Error!!!

declare function bar(obj: { readonly constructor: any }): void;

bar(Object) // OK

🙁 Actual behavior

ObjectConstructor is not a subtype of { readonly constructor?: any }, at the same time it is a subtype of a type { readonly constructor: any }

🙂 Expected behavior

In the code above, I expect that type ObjectConstructor will be a subtype of the object type { readonly constructor?: any }, because the object use constructor in a covariant way (I can't delete the property inside the function.)

[fatcerberus]

It's not a type error, per se. This is the clue:

Value of type 'ObjectConstructor' has no properties in common with type '{ readonly constructor?: any; }'.

This is due to weak type detection, which kicks in when the assignment target consists entirely of optional properties. That said, it still does seem to be a bug, since Object is assignable to { readonly constructor: any } and thus does at least have that property in common.

[andrewbranch]

Huhm, I’m not sure about this one. Object is of type ObjectConstructor which doesn’t say it has a constructor property; constructor properties just kind of magically show up on stuff, and I guess it has a similar carve-out for assignability of object-like types to { constructor: any }. But the code path that checks for common properties doesn’t pull in these magic properties. I guess it’s a bug.