Microsoft/TypeScript

Function implementation name must be ... (2389) when abstract method is declared again later

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#61,783 opened on May 29, 2025

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 (3 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: Error MessagesHelp WantedPossible Improvement

Description

🔎 Search Terms

duplicate abstract function declaration, abstract method, missing implementation, abstract method overload

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/IYIwzgLgTsDGEAJYBthjAgYge2wg3gFAIkKiQzwIBmArgHawAiApigIwAUAlAFwIA3bAEsAJgG5CxUnUasUAJh78Kw+gHMC00qSgsItKPQQByABbCT2hAF9r18tDiJZzNsi59BIiYTtA

💻 Code

abstract class Foo {
    abstract func1(): void;

    func2(): string {
    // ^? Function implementation name must be 'func1'.(2389)
        return 'hi'
    }
    
    abstract func1(): void;
}

🙁 Actual behavior

TS error reported for func2, which is valid.

🙂 Expected behavior

TS error reported for func1, which has duplicated abstract declaration.

Additional information about the issue

No response

Contributor guide

Tech stack
typescript
Domain
backend
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
understanding of TypeScript abstract classesbasic compiler architecture
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the TypeScript compiler source to locate where abstract method duplicate declarations are checked and where error 2389 is emitted. The bug suggests that when an abstract method is declared twice, the error is mistakenly reported on a subsequent non abstract method instead of the duplicate abstract method. Look at the checker.ts file, particularly the handling of abstract method declarations and the logic for error code 2389. The playground link provided reproduces the issue, so run the compiler locally with that code to trace the error emission. Check if there is any existing test for duplicate abstract methods and ensure the fix correctly identifies the duplicate abstract declaration.
Function implementation name must be ... (2389) when abstract method is declared again later · Microsoft/TypeScript#61783 | Good First Issue