improve `Unreachable case` error message

[He-Pin]

Compiler version

3.3.1

Minimized code

Not minimized yet.

1. clone incubator-pekko
2. change the scala3 version to 3.3.1
3. compile the multi-node-testkit with scala 3.3.1

Output

[error] -- [E030] Match case Unreachable Error: C:\Users\hepin\IdeaProjects\incubator-pekko\multi-node-testkit\src\main\scala\org\apache\pekko\remote\testconductor\Player.scala:218:14
[error] 218 |    case Event(_: ServerOp, _) =>
[error]     |         ^^^^^^^^^^^^^^^^^^^^^
[error]     |         Unreachable case
[error] one error found

Expectation

I just check the code, the branches will be matched with

    case Event(msg: NetworkOp, _) =>

first , so I think the compiler is right.

Better with an output:

[error] 218 |    case Event(_: ServerOp, _) =>
[error]     |         ^^^^^^^^^^^^^^^^^^^^^
[error]     |         Unreachable case, all case has been matched with `case Event(Done, _) =>` and `case Event(msg: NetworkOp, _)`
[error] one error found

[He-Pin]

@dwijnand I think the compiler is right, can you check it too, thanks.

[bishabosha]

I also checked this independently when I was testing pekko with pipeline compilation support, in the source file multi-node-testkit/src/main/scala/org/apache/pekko/remote/testconductor/DataTypes.scala all ServerOp also extended NetworkOp, so therefore would be covered by NetworkOp which is in the case above.

However for the project maintainers perhaps they should swap the cases.

[WojciechMazur]

I've tested it when investigating nightly failures in Open Community Build for akka/pekko and it's an improvement and not a regression. As @bishabosha already mentioned all the cases for ServerOp have been already covered and this code is in fact unreachable.

[He-Pin]

That's true.

[He-Pin]

Thank you everyone.

[He-Pin]

@bishabosha Is it possible to improve the error message, I mean you can reopen this issue with a different tile, or start a new issue.