This issue was picked for the Scala Issue Spree of tomorrow, Monday, January 13th. @jan-pieter, @nmcb and @RoccoMathijn will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

This will be @RoccoMathijn's first spree. Welcome! I would normally onboard new participants myself, but I would like to try something different today.

@jan-pieter and @nmcb, do you think you could help Rocco setting up his dev environment? Instructions are available at Getting Started and Setting up your IDE.

Then, it would be nice to quickly show him the main parts of the compiler:

• Compiler.scala that lists all the compiler phases,
• Trees.scala that defines the AST nodes used in the compiler (both typed and untyped),
• Types.scala that defines the types,
• Tests: positive tests are in tests/pos, negative tests are in tests/neg. They can be run from the SBT shell, either with scalac or testCompilation. Read more at Testing Your Changes.

About the issue itself, it seems to me that the check could be added in the RefChecks, a phase that does many well-formedness checks. The check might be added in RefChecks.checkExtensionMethods. You will need to add a new class in compiler/src/dotty/tools/dotc/reporting/messages.scala and report the error with report.warning or report.error—I am not sure if it should be a warning or an error, to be investigated.

If you have time, you could also look at #22232, which might be implemented in a similar way. Note the subtly different issue number 😄