Warn if multiple classes with the same qualified name exist

[asarkar]

This ticket arose out of scalatest/scalatest#2354.

I've this repo with the following structure:

.
├── graph
│   ├── src
│   │   └── package.scala
│   └── test
│       └── src
│           └── package.scala
└── build.mill

Both package.scala are in the package graph. While running tests, the src/package.scala is chosen by the classloader, causing a NoSuchMethodError (the method exists in the test/src/package.scala, and code compiles fine). Luckily, I was able to create a simple reproducer, and the folks at scalatest were kind enough to look into the issue, but in a larger project, this sort of inadvertent problems could be almost impossible to track down, especially because Scala classes and objects don’t have to follow the directory structure of their packages.

It was mentioned in the scalatest ticket that:

The class loader is created by the build tool sbt/mill

Hence, this ticket.

It'll be extremely helpful to have some sort of warning if multiple classes with the same qualified name exist, as well as a message showing which one is loaded. FWIW, there appears to exist a SBT plugin for this purpose.

[lefou]

Following your idea but without looking into the source file, there is a high chance of producing false warnings. Although equally named files could contain perfectly unambiguous content. The issue itself is also a runtime problem, not a compile-time problem. So to provide any useful insights, a full classpath scan would be required, which is an expensive operation. Nothing we would want to run before each run. Nevertheless, such tooling is possible and probably already exists. It would be best provided as some kind of diagnostics task to be used if there were run issues. I'm moving this into discussions for now.

[cheeseng]

@asarkar @lefou Sounds like CI/CD may be the right place to run the tool check.

[gamlerhart]

PS: I had some issue recently, where duplicated classes created link errors with ScalaJS.

However, I'm not sure what a good solution would look like. In the end the Link error is usually a hint enough for me to look into it.

[asarkar]

Not only CI, mill could show an error message on detecting NoSuchMethodError such as “These are often caused by classpath issues; try rerunning with —checkClasspath flag. Warning, it may take a while depending on the size of the project”

[cheeseng]

@asarkar It may be considered a scalac bug, not sure what's special in your case with given, but if you have 2 class with same name in main and test having different methods in them, calling any method for the class in main scalac will give you a compiler error, not runtime error. Unless there's a special reason for 'given' to do so, I don't see why it should have inconsistent behavior.

[lihaoyi]

I also recently hit issues with multiple classes (one defined upstream, one defined in the local project) with the same name accidentally colliding. Took me 1-2 hours to figure out. It doesn't happen often, but could definitely be nice to have some kind of warning or check to fail if these duplicate classfiles are detected