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[Release News] Scala v2.13.4 released!

Scala v2.13.4 has been released.

Scala 2.13.4:

Restores default global ExecutionContext to 2.12 behavior

Improves pattern matching, especially in exhaustivity checking

Adds experimental support for consuming some libraries built by Scala 3

and more! Details below.

Concurrency

NOTE The following change affects parallelism and performance. If you use scala.concurrent.ExecutionContext.global you may
want to adapt your code. (But note that Akka is unaffected, because it uses its own execution contexts.)

In 2.13.0 we made ExecutionContext.global “opportunistic”. This enabled “batching” of nested tasks
to execute on the same thread, avoiding an expensive context switch. That strategy requires
user code to wrap long-running and/or blocking tasks with blocking { ... } to maintain parallel
execution.

For 2.13.4, we restore 2.12’s default non-batching behavior, which is safer for arbitrary user code. Users wanting
increased performance may override the default, if they believe their code uses blocking correctly.
We make that choice available via ExecutionContext.opportunistic.

Using ExecutionContext.opportunistic requires a bit of extra boilerplate, made necessary by binary
compatibility constraints on the standard library. Detailed instructions are in
ExecutionContext.global’s Scaladoc.

Further detail: #9270/#9296/scala/bug#12089,

Pattern matching

The pattern matcher is now much better at warning you if a match isn’t exhaustive.

The following types of matches no longer disable exhaustivity checking:

guards (case <pattern> if <condition> => ...) #9140

custom extractors (user-defined unapply or unapplySeq) #9140/#9162

unsealed types, if you opt in via -Xlint or -Xlint:strict-unsealed-patmat #9140/#9299

Additionally,

private classes are now treated as if sealed #9211

singleton types no longer prematurely widen #9209

tuples are handled properly #9147/#9163/#9147

New warnings reported can be resolved by:

adding any missing cases

in the case of complementary guards (e.g. if n > 0 and if n <= 0) by dropping the last guard

for custom extractors: demarking irrefutable extractors as such, by defining the return type as Some

for sealed types: marking traits or parent classes sealed, parent classes abstract, and classes final

explicitly declaring the default case: case x => throw new MatchError(x)

Otherwise, your options for suppressing warnings include:

annotate the scrutinee with @unchecked, such as (foo: @unchecked) match { ... }

disable exhaustivity checking in the presence of guards and custom extractors with -Xnon-strict-patmat-analysis

disable exhaustivity checking of unsealed types with -Xlint:-strict-unsealed-patmat

use -Wconf to suppress the warnings globally, with e.g. -Wconf:msg=match may not be exhaustive:i

Scala 3 interop

This release enables the Scala 2 compiler to consume some libraries built in Scala 3. #9109/#9293

The new capability is experimental. To enable it, add -Ytasty-reader to your compiler options.

Not all Scala 3 built libraries are supported, because not all Scala 3 features can be supported.
The library author must stay within the supported subset.

For more details and caveats see the blog post Forward Compatibility for the Scala 3 Transition.

Standard library changes

When compiling on JDK 15, avoid clash with new CharSequence#isEmpty method #9292

The clash is avoided by making CharSequence wrappers in Predef non-implicit.

The change is binary compatible, but not source compatible. Call sites may need updating.

Make LazyList.cons.apply lazier #9095

Make MapView#values preserve laziness #9090

Make ListBuffer’s iterator fail when the buffer is mutated #9174

Un-deprecate useful StringOps methods, despite Unicode concerns #9246

Compiler changes

Allow using classOf with object type (e.g. classOf[Foo.type]) #9279

Fix back-quoted constructor params with identical prefixes #9008

Enable range positions (-Yrangepos) by default #9146

Compatibility

As usual for our minor releases, Scala 2.13.4 is binary-compatible with the whole Scala 2.13 series.

Upgrading from 2.12? Enable -Xmigration while upgrading to request migration advice from the compiler.

Contributors

A big thank you to everyone who’s helped improve Scala by reporting bugs, improving our documentation, spreading kindness in discussions around Scala, and submitting and reviewing pull requests! You are all magnificent.

This release was brought to you by 40 contributors, according to git shortlog -sn --no-merges HEAD ^v2.13.3 ^2.12.x. Thank you Jamie Thompson, Dale Wijnand, A. P. Marki, NthPortal, Lukas Rytz, Seth Tisue, Jason Zaugg, Georgi Krastev, Eugene Yokota, Martijn Hoekstra, Trey Cahill, Rado Buransky, Ergys Dona, Mike Skells, Greg Pfeil, Kazuhiro Sera, Mitsuhiro Shibuya, NagaChaitanya Vellanki, Sergei Petunin, Sébastien Doeraene, Takahashi Osamu, Viktor Klang, mwielocha, Nicolas Stucki, Jan Arne Sparka, Philippus Baalman, Glenn Liwanag, Rafał Sumisławski, Renato Cavalcanti, Sergei, nooberfsh, Dmitrii Naumenko, Simão Martins, counter2015, Jian Lan, Liu Fengyun, Kanishka, Julien Richard-Foy, Janek Bogucki, Björn Regnell.

Thanks to Lightbend for their continued sponsorship of the Scala core team’s efforts. Lightbend offers commercial support for Scala.

Scala 2.13 notes

The release notes for Scala 2.13.0 have important information applicable to the whole 2.13 series.

Obtaining Scala

Scala releases are available through a variety of channels, including (but not limited to):

Bump the scalaVersion setting in your sbt-based project

Download a distribution from scala-lang.org

Obtain JARs via Maven Central