Negated {} and null | undefined predicates on unknown should still narrow #60789

🔎 Search Terms

{} undefined null predicate unknown narrowing

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about narrowing

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXywGcA5HDY5CCIgCwAoA3KCZEALnjQGtUcB3VAEoOTFgiLwA3gF8A3AChQkWAhTpseAiUrVC9Uaw7deA4fAPjC8VDvgAfTqlCIsqEMAWLw0OPDB5CDHNmQ0ceflRPLER4OiJSch1aRhCQQUEpeXhgsQVsgHp8+AA9AH55aXl5aNiAQniyCipki3TM7Is8+EKS8srqmLjtZr0UsTbJLJzWLp6yiqqaunrh3X1UianOqbm+oA

💻 Code

declare function isNotNullish(value: unknown): value is {};
declare function isNullish(value: unknown): value is null | undefined;

declare const value: unknown;

if (isNotNullish(value)) {
  value;
  // ^?
}

if (!isNotNullish(value)) {
  value;
  // ^?
}

if (isNullish(value)) {
  value;
  // ^?
}

if (!isNullish(value)) {
  value;
  // ^?
}

🙁 Actual behavior

value only narrows when the type predicate is called directly, not when negated with a !.

The narrowed types of value in that example are:

1. isNotNullish(value): {}
2. !isNotNullish(value): unknown ❌
3. isNullish(value): null undefined
4. !isNullish(value): unknown ❌

🙂 Expected behavior

The narrowed types of value should be:

1. isNotNullish(value): {}
2. !isNotNullish(value): null | undefined
3. isNullish(value): null undefined
4. !isNullish(value): {}

Additional information about the issue

No response