Reports Scala 2 syntax that can be replaced with Scala 3 when using -Xsource:3.

Under the compiler flag -Xsource:3 the following Scala 3 constructs are already supported in Scala 2.13.6:

• ? instead of _ as wildcard marker in wildcard types
• case in pattern bindings of for-comprehensions
• * instead of _ in imports
• as soft-keyword instead of => in import aliases
• open and infix modifiers are allowed
• New Scala 3 vararg splices syntax
• & instead of with in compound types