Microsoft/TypeScript

Incorrectly reported `TS2322` error on array items after spread

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#55,340 opened on Aug 11, 2023

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BugDomain: Error MessagesHelp Wanted

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Description

🔎 Search Terms

"array spread", "TS2322"

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgLhgQQE6rgTwDxgK4C2ARgKaowA+M0qAlmAOYB8MAvDANoC6A3AFChIsIkjQYc+YmRbsOAOgVwANDACMvIA

💻 Code

const a: Array<number | string> = [];
const b: Array<number> = [...a, 1];

🙁 Actual behavior

The compiler gives the following two errors, of which only one is appropriate:

~/prog/typescript ❯ tsc ./array-spread-error.ts
array-spread-error.ts:2:27 - error TS2322: Type 'string | number' is not assignable to type 'number'.
  Type 'string' is not assignable to type 'number'.

2 const b: Array<number> = [...a, 1];
                            ~~~~

array-spread-error.ts:2:33 - error TS2322: Type 'string | number' is not assignable to type 'number'.
  Type 'string' is not assignable to type 'number'.

2 const b: Array<number> = [...a, 1];
                                  ~

🙂 Expected behavior

The spread is clearly incorrect, as the original array could potentially contain strings as well, but the number literal should not be marked with the same error.

Contributor guide

Research direction
Investigate how TypeScript handles array spread in type checking. The error on the spread element (position 0) is correct, but the error on the literal '1' (position 1) is incorrect. The fix likely involves adjusting the error location assignment in the checker or the logic that propagates types in array literals.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitTypeScript basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70