Nullish coalescing operator does not understand cases where at least one operand is guaranteed to not be null, but not which one is not null

[RubyTunaley]

🔎 Search Terms

"nullish coalescing" "narrowing" "type narrowing" "if statement" "or expression"

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code: https://www.typescriptlang.org/play/?target=6&ts=5.9.0-dev.20250507#code/MYewdgzgLgBAhgLhmArgWwEYFMBOMA+yKANsTALwwAM8E8YAngNwBQoksGSqmuBRpCtVr1mLAJYAzGAAo4MAITlKqQfkIZFygcQCUMAN4sAkO2gxg3dNjyV5AfnswMrAL4sgA

💻 Code

const a: number | null = 0 as any;
const b: number | null = 0 as any;
if (a !== null || b !== null) {
	const c: number = a ?? b;
}

🙁 Actual behavior

The type checker generates an error when assigning to c: Type 'number | null' is not assignable to type 'number'.
Type 'null' is not assignable to type 'number'.

This is incorrect because the if statement guarantees that at least one of a or b is not null.

This also happens with code that excludes rather than narrows, e.g.:

const a: number | null = 0 as any;
const b: number | null = 0 as any;
if (a === null && b === null) {
	throw new Error();
}
const c: number = a ?? b;

🙂 Expected behavior

No error should occur because the if statement guarantees that at least one of a or b is not null. If the code was modified so that an explicit type was not specified, c should be inferred as number.

Additional information about the issue

No response

[MartinJohns]

Duplicate of #49452 and a few others. Search terms: coalesce in:title

[RyanCavanaugh]

See #61578, #48536, etc -- narrowing isn't equipped to do the sort of constraint solving exercise this code pattern would require.

[typescript-bot]

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