Bad diagnostic when inheriting from class in extension

[AnthonyLatsis]

The following errors are misleading and could use a rewording as clear language rules, depending on the kinds of extended and inherited types.

protocol P {}
struct S {}
class C1 {}
class C2 {}

extension C2: S {} // error: inheritance from non-protocol type 'S'
extension C2: C1 {} // error: inheritance from non-protocol type 'C1'
extension C2: C1 & P {} // error: inheritance from class-constrained protocol composition type 'C1 & P'

Environment

• Swift version 5.9-dev (LLVM 6a04c4848990c70, Swift c1d5118)

NB: How to improve on these is left as an exercise to whoever takes the issue.

[StevenWong12]

Hey all, I'd like to give a try on solving this issue.😀

[Nallu-Swami]

Hello, can I work on this issue if no progress has been made yet for this issue?

[AnthonyLatsis]

@StevenWong12 Are you working on this?

[StevenWong12]

@Aringopalan Please feel free to take it😊

[Nallu-Swami]

Thanks a lot @AnthonyLatsis sir

[Nallu-Swami]

im really sorry i did that by mistake could you pls assign it back to me 😅

[Nallu-Swami]

The following errors are misleading and could use a rewording as clear language rules, depending on the kinds of extended and inherited types.

protocol P {}
struct S {}
class C1 {}
class C2 {}

extension C2: S {} // error: inheritance from non-protocol type 'S'
extension C2: C1 {} // error: inheritance from non-protocol type 'C1'
extension C2: C1 & P {} // error: inheritance from class-constrained protocol composition type 'C1 & P'

Environment

• Swift version 5.9-dev (LLVM 6a04c4848990c70, Swift f2bac1a22939a4d)

NB: How to improve on these is left as an exercise to whoever takes the issue.

Reviewing the code and after several tries i believe this to be one optimal rewording

protocol P {}
protocol S {}
class C1 {}
class C2class {}

extension C2class: S {}
class C2: C1 {} 
extension C2class:P {}

[Nallu-Swami]

If there is any further issue with the code ill work on it and reword it for better compilation

[StevenWong12]

I think the original vision of this issue is to rewrite the diagnostic message in the type check stage e.g.
error: class can only inherit from protocol in extension declaration

[Nallu-Swami]

I think the original vision of this issue is to rewrite the diagnostic message in the type check stage e.g.

error: class can only inherit from protocol in extension declaration

Ohhh 🫠, I did a total 360 here but I guess I can still work on it, rephrase all of them to be more readable.

[dk-talks]

Hey @Aringopalan , Are you still working on the issue?

[Nallu-Swami]

@dk-talks feel free to take it , im not working it.

[dk-talks]

Hey @AnthonyLatsis I have resolved this issues. Please have a look at my Pull Request. #65531

[GunjanRawat26]

@AnthonyLatsis is this issue still open?

[AnthonyLatsis]

@GunjanRawat26 Yep, errors are still the same.

[GunjanRawat26]

@xedin Hey, I’m planning to work on this issue, and I noticed that in a previously raised PR, you mentioned that only certain types are allowed in the inheritance clause here. However, it seems like that case was already handled here. Could you please brief me on any additional cases that still need to be addressed?
Also can we phrase the error something as error: class can only inherit from protocol in extension declaration

[AnthonyLatsis]

@GunjanRawat26 I recommend focusing just on the cases shown in the original example.

Also can we phrase the error something as error: class can only inherit from protocol in extension declaration.

This can be interpreted as "a class can inherit from protocol only in extension", which is wrong. I think we can do better. For example: error: extension may declare inheritance only from protocols + note: 'X' is not a protocol.