False positive ts(2373)

🔎 Search Terms

Parameter cannot reference identifier declared after it, ts(2373)

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about anything related (and found nothing).

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&noUnusedLocals=true&noUnusedParameters=true&esModuleInterop=false&declaration=false&target=99&jsx=0&noErrorTruncation=true&pretty=true&isolatedDeclarations=true&verbatimModuleSyntax=true&useUnknownInCatchVariables=true&exactOptionalPropertyTypes=true&noFallthroughCasesInSwitch=true&noImplicitOverride=true&noPropertyAccessFromIndexSignature=true&ts=5.6.0-dev.20240606&ssl=18&ssc=1&pln=1&pc=1#code/PQKhCgAIUgVALAlgZ0gMwK4DsDGAXRAey0j3gCdCB3VAAwCUBTNR8x3RgUXMvIC5IAYQCGWLITyRhOHI2SoA5ABt2AczILIAI2aE2kRFkQFhSxAC9hBYrSmTy2AgFtGAOigRocAMqQcetnwlAE9INgAHPTw6aIAKACYAZgB2RIBKWldIQF4NwFkdj2BwRgAPSPJJfyxkSTwHRgAFQmRjRAA3RkgAXkgAHkrquAA+WIBvSABtFSx1eEgAWkgARgBdATRDUwBJPEYnABpIKZnIAF8BNmEAE2IQuHHltIFYSAAfSGxL5kNGS67ByBGUDCjDwGHIJHWWC2OycAG5wCd4eBQJ4YAgUOhsPgiCQqHoANaoSEdKxhRyIFzuaAeHzQECGMD+HiMIKhCJRGLIBIpdKZHKAOR2CkVSlE-MQBmhTMgGk0Wu0ur1+pJYMMxkcyAdJmoyPMlqt0BslNtdqdzowrjdQrB7o84K93lhPsTfp1-oDIMDQeCDVCjTD4YjwEA

💻 Code

/**
 * This function throws `ReferenceError: Cannot access 'length' before initialization` at runtime.
 *
 * TS correctly reports `ts(2373)`. 👍
 */
export const truePositive = <const T>({ [length - 1]: finalItem, length }: readonly T[]): T | undefined => {
  return finalItem;
};

/**
 * This function works fine at runtime.
 *
 * TS **in**correctly reports `ts(2373)`. 👎
 */
export const falsePositive = <const T>({ length, [length - 1]: finalItem }: readonly T[]): T | undefined => {
  return finalItem;
};

🙁 Actual behavior

A false positive ts(2373) is being reported in a situation where we can determine statically that what the report warns about is not taking place.

Ironically, the false positive ts(2373) might represent a real bug in TS' own modeling of declaration order.

🙂 Expected behavior

No false positive ts(2373) being reported.

Additional information about the issue

To be clear:

In the falsePositive case, we can statically determine that length is declared prior to being used.

Start with the linked TypeScript Playground reproduction and compare the truePositive and falsePositive parameter patterns. Trace the implementation of diagnostic ts(2373), then add a regression test for the shown declaration-order case. Done means the falsePositive example produces no diagnostic while truePositive still reports the runtime hazard.