Subclass of a (non-abstract class that uses declaration merging to implement parent abstract member) also has to implement same abstract member

🔎 Search Terms

subclass inherit abstract concrete method property member complains parent declaration merge merging satisfy implement

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.3#code/JYOwLgpgTgZghgYwgAgApyhcBJZBvAKGWORgHsyAKASgC5kA3M4AEwIF8C4AjAZzCiIwyBABs4vXmgxZhhEsh79BCYeSp1GzNpwIB6PcgCyrFqJTAAtgAdzl2VPSZwjYHGQsIYjHDDAyIMj2UADmEAQE3pLGpubIEAAekCAsjjIueJygkLCIKCYsZiiJyanSzmC4mRFRUgDCABbAoizxSVhlBUX47MgGHmQQvCAA5MIA7mRQANYR-V1xVrYQ9uBpFR7AmKqiAJ6R4tELEABMbaXrsvhEJOo0PRw1h-VNLWclHVLHZ5l9hpMzXhAA

💻 Code

interface ParentI {
    foo(): void
}
abstract class Parent {
    abstract foo(): void
}

// Middle implements Parent via declaration merge

class Middle extends Parent {}
interface Middle extends ParentI {}

class Child extends Middle {} // doesn't work

// Middle implements Parent directly
class Middle2 extends Parent {
    foo() {}
}

class Child2 extends Middle2 {} // works

🙁 Actual behavior

Typescript complains that Child must implement an foo, even though Middle has already implemented foo via declaration merging.

🙂 Expected behavior

I would expect Child to pass type-checking, just as Child2 does. Seeing as how Middle (a concrete class) is type-safe and Child extends Middle with no further modification, this feels very surprising.

Additional information about the issue

No response

Start with the provided TypeScript Playground reproduction and compare the declaration-merging example with the directly implemented Middle2 case. Trace the compiler's handling of abstract members across merged class and interface declarations, then add a regression test showing that Child type-checks without implementing foo itself.