A2 Scala 2 Compatibility

Existential Types

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

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.

Compound Types (with)

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.