Binary expression in an `if` condition can't narrow identifier reassigned within expression of the optional chain

🔎 Search Terms

binary condition refinement predicate optional chain undefined

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20240109#code/C4TwDgpgBAKu0F4oG8BQUoEsAmAuKAdgK4C2ARhAE4DcqAvraqJFAJIEBmVEBAxhOw4B7KEjQZmEAAoBDSjJIRgVfHEi0MmEmAA2mCNgCClTKAD8+YuSq0GqVNgi8dc6ByJ9gmIQSgBzJUFuPgFOIQAxIUo1CAAKSVV4AEp8IMoefkERAB8oD0cOTAIDRndPb18OAgBGeOrEyAAaKGAAJgaIJJR0KB0lLDDRf0DOYMywyOj4OqSNLA4oWKLhMwA6LV19IxNQKABCBCR8iELi7C7xDAHhde09A2NTEDm6KAgdAGdoTAXYpcGkAFgGkMqFhJMYvFWkkkmsNvdtk99oc8gQCkUDBcepowrdNg8ds8enR6PYyrwvD4oFVWnUOs02h0sRg+sBriJASMuOkQlkIdNgNVZj0fotlkI4Xcto9dgcjmiThjzt0ruy8QiZUSMK93l95ot-sIhkCQbyJlFIW0YZL8YjQMyruL1dLCS9SUA

💻 Code

type Type = {
  id: number;
};

type InferenceInfo = {
  typeParameter: Type;
  impliedArity?: number;
};

declare function getInferenceInfoForType(type: Type): InferenceInfo | undefined;

function fn1(t1: Type, t2: Type) {
  let info = getInferenceInfoForType(t1);
  if (info?.impliedArity !== undefined) {
    info.impliedArity; // ok
  } else if ((info = getInferenceInfoForType(t2))?.impliedArity !== undefined) {
    info.impliedArity; // 'info' is possibly 'undefined'.(18048)
  }
}

function fn2(t1: Type, t2: Type) {
  let info = getInferenceInfoForType(t1);
  if (info?.impliedArity !== undefined) {
    info.impliedArity; // ok
  } else if ((info = getInferenceInfoForType(t2))?.impliedArity) {
    info.impliedArity; // ok
  }
}

🙁 Actual behavior

The second branch in fn1 contains an error

🙂 Expected behavior

I'd expect this to be narrowed just fine - like the first branch and the second branch in fn2

Additional information about the issue

No response

Start with the linked TypeScript Playground reproduction and compare the narrowing behavior in fn1 and fn2, especially the reassigned optional-chain expression. Trace the type-checking path for the second branch; done means fn1 accepts the access to info.impliedArity without an error while preserving the existing checks.