🔎 Search Terms

comma operator nullish coalescing never

🕗 Version & Regression Information

• This changed between versions 5.5.4 and 5.6.3

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.3#code/FAEwpgxgNghgTmABBA9gOwM4BdEFsYCeARmAFyJZwCuSAPomlVFANzDBRg4CWiAvIgAMbYAG9giScnTZEMfnkIlEAfhUVqYNoglTUmHEQUAKbgGozAGkXEwASm16ZOCAqNqNNEVOkHEIE3MrGxI7VXVKL2AAX3ZgIA

💻 Code

declare const maybe: true | null;

let i = 0;

// This works
const a = maybe ?? true;

// This works
const b = (i++, maybe);
const c = b ?? true;

// This fails
const d = (i++, maybe) ?? true;

🙁 Actual behavior

A build error occurs:

Right operand of ?? is unreachable because the left operand is never nullish. ts(2869)

🙂 Expected behavior

The code should compile without error; The left side of the ?? can still be nullish in the const d case.

The const b / const c case is functionally identical and is recognized to be valid

Additional information about the issue

No response