JEP 390: Warnings for Value-Based Classes

[xuwei-k]

Reproduction steps

Scala version: 2.13.18

Welcome to Scala 2.13.18 (OpenJDK 64-Bit Server VM, Java 21.0.12).
Type in expressions for evaluation. Or try :help.

scala> val a = java.time.LocalDate.now()
val a: java.time.LocalDate = 2026-09-20

scala> a.synchronized(a)
val res0: java.time.LocalDate = 2026-09-20

Problem

expect warning same as Java

$ jshell
|  Welcome to JShell -- Version 21.0.12
|  For an introduction type: /help intro

jshell> var a = java.time.LocalDate.now()
a ==> 2026-09-20

jshell> int b() { synchronized(a) { return 2; } }
|  Warning:
|  attempt to synchronize on an instance of a value-based class
|  int b() { synchronized(a) { return 2; } }
|            ^---------------------------^
|  created method b()

Note

• JEP 390: Warnings for Value-Based Classes scala3#27110
• https://openjdk.org/jeps/390