'Types have no overlap' when extending Partial of generic type

[vostrnad]

Bug Report

When comparing two variables of generic types, one of which extends the Partial of the other, TypeScript complains that the types have no overlap.

🔎 Search Terms

TS2367 partial generic

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type System Behavior, Generics

⏯ Playground Link

Playground link with relevant code

💻 Code

const equal = <T, P extends Partial<T>>(item: T, partial: P) => {
  // incomplete implementation
  return item === partial // error
}

🙁 Actual behavior

The code above produces this error:
This condition will always return 'false' since the types 'T' and 'P' have no overlap.

🙂 Expected behavior

Since the types T and Partial<T> do overlap, I would expect the above code to compile without errors.

[RyanCavanaugh]

Q: Why isn't the signature written this way?

const equal = <T,>(item: T, partial: Partial<T>) => {
  return item === partial // no error
}

The comparability check is necessarily incomplete (meaning errors more than it "should") to do anything useful; it's true that there's no assertion route from T to P here but that follows from our other rules. I'm not sure there's an improvement to be found here that doesn't make the comparability rule more confusing than it already is

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.