Error (2320) when extending interfaces where a property is both readonly and readwrite.

🔎 Search Terms

Related issues:

• https://github.com/microsoft/TypeScript/issues/4278
• https://github.com/microsoft/TypeScript/issues/13347

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground Link

💻 Code

interface A { readonly foo: number; }
interface B { foo: number; }

interface C extends A, B {}

🙁 Actual behavior

Interface 'C' cannot simultaneously extend types 'A' and 'B'.
  Named property 'foo' of types 'A' and 'B' are not identical.(2320)

🙂 Expected behavior

No error.

Additional information about the issue

As far as I understand, readonly isn't a constraint, but an interface specification.
Meaning, that readonly foo: number is almost equivalent to:

get foo(): number;

Not to:

get foo(): number;
set foo(_: never): void; // If I'm not mistaken this should forbid assignations.

(TBH, a read keyword would be less confusing than readonly.)

Therefore, here, I want to merge:

get foo(): number;

With:

get  foo(): number;
set foo(f: number): void;

We can see it as:

• "merging" 2 identical getters, which cause no issue.
• adding an additional property (like we could add a method) : the setter.

However, the latter causes an issue when it shouldn't. This is just adding something.

Note: TS requires the 2 types of foo to be strictly identical.
But, when one is readonly, then it could accept the second to be a specialization of it (i.e. assignable to the readonly version).

Indeed, the resulting interface would still be assignable to the first one.

Beginne mit dem verlinkten Playground-Beispiel und vergleiche das in den verwandten Issues 4278 und 13347 beschriebene Verhalten. Untersuche die Behandlung der Schnittstellenkompatibilität für readonly- und readwrite-Eigenschaften; abgeschlossen ist die Arbeit, wenn das Beispiel nicht mehr den Fehler 2320 meldet, während das bestehende Kompatibilitätsverhalten korrekt bleibt.