You can setup auto-update of Scala plugin Nightly builds in IntelliJ IDEA.
Simply select desired update channel from a drop-down menu at "Updates" tab in Scala plugin settings page.

• SCL-9239 A lot of sub-projects in Multi-project build
• SCL-9287 Missing specs2 and junit libraries in the dependencies of a new Play project
• SCL-9309 No warning for duplicate aliases
• SCL-9263 Value classes are not highlighted
• SCL-9317 Mark expanded macro applications
• SCL-2066 Formatter incorrectly indents braces of methods and other blocks if they are configured to be on a new line.
• SCL-8959 Change "working directory" of scala compiler to match project home
• SCL-9272 Labeled objects do not work in "Evaluate expression" if "Scala" is selected
• SCL-9124 Scala plugin shows too many duplicated classes in inheritors dialog
• SCL-9294 With Scala plugin enabled, com.intellij.debugger.NoDataException is thrown while debugging
• SCL-2507 default parameters should drive type inference
• SCL-9273 Hierarchy, Go to implementation, and implementation gutter icons are broken
• SCL-9290 NPE on running Play
• SCL-7178 Provide better error message, when two type have the same name
• SCL-7948 When accessing Scala's Tuple2 elements, ._1 and ._2 are highlighted as invalid syntax. I need to use ._1() and ._2().
• SCL-8195 An ability to mark some imports as used (so they are not removed upon reformat)

• SCL-9235 Incorrect "Java accessor method called as empty-paren" with Java 8 Stream API
• SCL-8167 Perform eviction for dependencies when importing project

• SCL-8800 Code highlighting is totally broken for simulacrum

• SCL-9270 possibility to create and use variables in evaluate expression
• SCL-9240 Good code yellow: "Useless expression" flagged incorrectly
• SCL-9179 Make 'Enable optimize imports automatically' command not a top-most one

• SCL-8933 Good code red: Wrong type inference in for comprehension (inferes to Nothing)

• SCL-9252 add quickfix for "Type annotation required" inspection

• SCL-8866 Syntax highlighting issue with Java 8

• SCL-9018 EAP Debugger do not respect breakpoints in tests
• SCL-9141 Debugger jumps around in match statement
• SCL-9049 Scaladoc generates stub with return also when return type is Unit
• SCL-9031 Inspection for mapping to unit values fires when mapping to function returning unit
• SCL-9046 Play Framework, Twirl: Syntax highlighting error.
• SCL-9051 Cannot rename variable on twirl parameters
• SCL-9045 Play Framework, Twirl: Syntax highlighting error. Assigning an expression to an html attribute.
• SCL-9230 debugger trees: label don't changes from "Collecting data..." for vars
• SCL-8965 SBT import is still broken (two months already)
• SCL-7909 incorrect "unnecessary parentheses" hint in scala
• SCL-3520 Honour "skip simple getters" setting
• SCL-9191 Throwable on new Play project creation
• SCL-9010 "Your IDEA is outdated to use with Scala plugin" message with reference to build already in use
• SCL-9192 Expression of type BigDecimal doesn't conform to expected type BigDecimal

• SCL-3520 Honour "skip simple getters" setting

• SCL-9141 Debugger jumps around in match statement
• SCL-7909 incorrect "unnecessary parentheses" hint in scala

• SCL-9175 ScalaDoc parsing fails with code block containing JSON
• SCL-9157 Faster file type detection

• SCL-3151 widen selection should include XML attribute text, then attribute text with quotation marks
• SCL-9158 Scala getClassesByName takes too much in a project that has no Scala
• SCL-4953 parameter info doesn't find apply method of compound type
• SCL-5437 Inspection idea: warn when selecting a 'val' from a DelayedInit subclass (e.g. App)

• SCL-7431 Play template variables that generate full HTML attributes marked as errors
• SCL-9159 Inefficient ScClsStubBuilder
• SCL-8966 Scala worksheet doesn't work with backticks

• SCL-4484 Mark "then" as deprecated

• SCL-9146 ScalaJS React <. broken in Intellij 15 EAP

• SCL-9147 Stuck in an infinite loop when trying to import a project

• SCL-8989 Unable to track types through Scala Graph nodes
• SCL-8486 Copy+paste loses formatting JS code in *.scala.html template
• SCL-6738 Javascript formatting broken in Play Framework Scala Templates

• SCL-9129 some files from scala library are decompiled as java in scala 2.12.0-M2
• SCL-9135 Generate property generates wrong name for a setter

• SCL-9121 Import statements in worksheet become duplicated in output
• SCL-3603 good code red
• SCL-9124 Scala plugin shows too many duplicated classes in inheritors dialog

• SCL-5099 Pattern variables in field declarations are highlighted as fields

• SCL-9100 Create run configuration for Play pfoject
• SCL-9102 Open in browser after compilation is not working

• SCL-9044 SBT 0.13.9 not working with Intellij

• SCL-9046 Play Framework, Twirl: Syntax highlighting error.
• SCL-9049 Scaladoc generates stub with return also when return type is Unit
• SCL-9051 Cannot rename variable on twirl parameters

• SCL-9015 Very usual situation, when class can't be resolved in evaluate expression
• SCL-9019 A jar dependency (httpcore 4.0.1) keeps being "detached" from its "library" in the IDEA project every time the Gradle project is refreshed

• SCL-8999 Feature needed for call-by-name value parameter error
• SCL-9006 IAE at ScTypePsiTypeBridge$class.create() on Copy of Java type specification with additional bound

• SCL-9004 Interpolation completion is not working for function parameter
• SCL-8944 Update ScalaStyle Version

• SCL-8875 Good code red: weird type inference issues

• SCL-8995 Good Code Red: Cannot resolve method Xxx.Unapply where the unapply method is synthetic but overrides an abstract unapply method
• SCL-8932 Invalid Replace with Option application when body is a function call

• SCL-8579 SBT import does not respect JDK deduced by sbt-structure 1.4.15

• SCL-8965 SBT import is still broken (two months already)

• SCL-8631 Type error not shown when type tooltip is enabled

• SCL-8162 Support Kind Projector

• SCL-8890 inspection access to first/last element by index
• SCL-8917 Breakpoints in lambda sometimes trigger in a different source file
• SCL-8921 Don't truncate the logger message in org.jetbrains.jps.cmdline.BuildMain (line 405?)

• SCL-8935 NPE at autocompletion

• SCL-8920 New SAM inspection doesn't catch Callable
• SCL-5367 Good code shows as red: Using Value Types from Java

• SCL-8157 Good code red: calling apply in implicitly converted object

• SCL-8908 Plugin version lost if switching update channel

• SCL-8793 Good code red: function resolves to a completely unrelated library
• SCL-7170 Good code red: underscore after a by-name parameter
• SCL-7885 Java editor doesn't recognize all exceptions marked with @throws in Scala code

• SCL-4625 Spurious error accessing a protected method within a java class

• IDEA-142373 JavaTestFinders escape

• SCL-8863 Good code red: @uncheckedVariance annotation
• SCL-6140 Support Scala 2.11 SAM

• SCL-8803 Error highlighted when using multiple constructors for covariant generics
• SCL-8632 Import quick fix menu automatically open Exclude sub-menu

• SCL-8760 Wrong documentation is shown in editor
• SCL-8727 Play 2.4 RC2 Routes error highlighted (routesGenerator)

• SCL-8600 <Label>_DebugLabel variables not accessible in Scala debugger

• SCL-8568 Disable "Name boolean parameters" for single argument methods

• SCL-8626 Noinspection ScalaFileName is not working
• SCL-8659 @Nullable parameter annotation in Scala ignored in Java
• SCL-8704 Wrong "Convert match statement to pattern matching anonymous function" suggestion
• SCL-8623 Introduce variable: wrong place is chosen in for statement