Unexpected type mismatch error for Either and Either.Right.

[ikru]

VSCode Version: 1.99.3
macOS: 26.0 (25A354)
Language Support for Java(TM) by Red Hat v1.45.0
Java Version: Zulu 17.60 (but I was able to repro on multiple versions

vs code won't build the project reporting the code issue as Type mismatch for Either vs Either.Right. Either.Right is a derived class from Either.
When building with mvn compile or with other IDEs this code works just fine.

Steps to Reproduce:

1. Create a function

    public <R> Mono<Either<Throwable, ResponseEntity<List<SimpleProductTileV2>>>> noOfferProductDetails(
            BffRequestContext<R> requestContext) {
        
        // No liam return empty list of products
        return Mono.just(new Either.Right<>(new ResponseEntity<>(Collections.<SimpleProductTileV2>emptyList(), HttpStatus.OK)));
    }

2. Build
3. Get error

Type mismatch: cannot convert from Mono<Either.Right<ResponseEntity<List<SimpleProductTileV2>>>> to Mono<Either<Throwable,ResponseEntity<List<SimpleProductTileV2>>>>

mvn compile works just fine

Looks like vs code compiler incorrectly decode Either.Right or something like that.

[fbricon]

try enabling the javac compiler in your settings.json:

"java.jdt.ls.javac.enabled": "on"

[ikru]

Thanks @fbricon, I am using Java 17, enabling javac tells me Requires running this extension with Java 24.

[fbricon]

You need Java 24 to run the Java language server, to enable the javac compiler, but you can still use Java 17 to compile your project. See https://github.com/redhat-developer/vscode-java/wiki/JDK-Requirements#project-jdks

[ikru]

Right, and why would Java 17 fail this cast in vscode, while it works with mvn compile? And if I replace it with implicit cast, it works in vs code too.

Either<Throwable, ResponseEntity<List<SimpleProductTileV2>>> response =
            new Either.Right<>(new ResponseEntity<>(Collections.<SimpleProductTileV2>emptyList(), HttpStatus.OK));
        return Mono.just(response);

[fbricon]

because by default, vscode-java relies on the ECJ compiler, which is a different implementation from javac. Sometimes discrepancies between the compilers can occur, unfortunately.
@jjohnstn is this a JDT issue you're aware of?

[jjohnstn]

@fbricon Not that I am aware of. I suggest opening a JDT core issue.

[fbricon]

upstream issue: eclipse-jdt/eclipse.jdt.core#4469