Existential types using forSome (as in SLS §3.2.12) are not available in Scala 3.
Therefore when reading an existential type from Scala 2, the following happens:
Existential types that can be expressed using only wildcards (but not
forSome) are treated as refined types.
For instance, the type
Map[_ <: AnyRef, Int]is treated as the type Map, where the first type parameter
is upper-bounded by AnyRef and the second type parameter is an alias
of Int.
When reading class files compiled with Scala 2, Scala 3 will do a best effort to approximate existential types with its own types. It will issue a warning that a precise emulation is not possible.
Procedure syntax
def f() { ... }has been dropped. You need to write one of the following instead:
def f() = { ... }
def f(): Unit = { ... }Scala 3 accepts the old syntax under the -source:3.0-migration option.
If the -migration option is set, it can even rewrite old syntax to new.
The Scalafix tool also
can rewrite procedure syntax to make it Scala 3 compatible.
Intersection types A & B replace compound types A with B in Scala 2.
For the moment, the syntax A with B is still allowed and interpreted as A & B, but its usage as a type (as opposed to in a new or extends clause) will be deprecated and removed in the future.