Report a clearer error message for unexpected `final` modifier

[som-snytt]

Welcome to Scala 3.3.1-RC1-bin-SNAPSHOT-git-5262680 (20.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> final val x = 42
-- [E088] Syntax Error: ------------------------------------------------------------------------------------------------
1 |final val x = 42
  |      ^^^
  |      Expected start of definition
  |
  | longer explanation available when compiling with `-explain`

Expectation

That seems like a natural thing to do in the REPL.

[mbovel]

Scala 2:

$ scala-cli repl -S 2.13
Welcome to Scala 2.13.14 (OpenJDK 64-Bit Server VM, Java 17.0.8).
Type in expressions for evaluation. Or try :help.

scala> final val x = 42
val x: Int(42) = 42

scala> val x = 42
val x: Int = 42

Scala 3:

$ scala-cli repl -S 3.4
Welcome to Scala 3.4.2 (17.0.8, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                
scala> final val x = 42
-- [E088] Syntax Error: --------------------------------------------------------
1 |final val x = 42
  |      ^^^
  |      Expected start of definition
  |
  | longer explanation available when compiling with `-explain`
scala> final def x = 42
-- [E088] Syntax Error: --------------------------------------------------------
1 |final def x = 42
  |      ^^^
  |      Expected start of definition
  |
  | longer explanation available when compiling with `-explain`

Why should this work; shouldn't final only be available inside classes?

What is the use-case for this?

[mbovel]

Note: you get the same error when trying to define a final val in a method for example:

➜  ~/scala-snippets-5 cat finalVal.scala
package finalVal

@main def Main =
  final val x = 42
  ()
➜  ~/scala-snippets-5 scala-cli run finalVal.scala
Compiling project (Scala 3.4.1, JVM (17))
[error] ./finalVal.scala:4:9
[error] Expected start of definition
[error]   final val x = 42
[error]         ^^^
Error compiling project (Scala 3.4.1, JVM (17))
Compilation failed

[mbovel]

Note from @dwijnand: -explain actually doesn't show anything useful here.

[mbovel]

This issue has been re-purposed to making the error message clearer. Note that this will not be specific to the REPL.

[mbovel]

This issue was picked for the Scala Issue Spree of today, June 11th. @aherlihy, @mbovel, @bracevac, @EugeneFlesselle will be working on it.