Add an error message for local final defs

Co-Authored-By: Eugene Flesselle <[email protected]>
Co-Authored-By: anna herlihy <[email protected]>
Co-Authored-By: Oliver Bračevac <[email protected]>

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -4560,6 +4560,7 @@ object Parsers {
       for (imod <- implicitMods.mods) mods = addMod(mods, imod)
       if (mods.is(Final))
         // A final modifier means the local definition is "class-like".  // FIXME: Deal with modifiers separately
+        if isDclIntro && in.token != GIVEN then syntaxError(FinalLocalDef())
         tmplDef(start, mods)
       else
         defOrDcl(start, mods)

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -212,6 +212,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ContextBoundCompanionNotValueID // errorNumber: 196
   case InlinedAnonClassWarningID // errorNumber: 197
   case UnusedSymbolID // errorNumber: 198
+  case FinalLocalDefID // errorNumber: 199
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1850,6 +1850,12 @@ class ExpectedStartOfTopLevelDefinition()(using Context)
     i"You have to provide either ${hl("class")}, ${hl("trait")}, ${hl("object")}, or ${hl("enum")} definitions after modifiers"
 }
 
+class FinalLocalDef()(using Context)
+  extends SyntaxMsg(FinalLocalDefID) {
+  def msg(using Context) = i"The ${hl("final")} modifier is not allowed on local definitions"
+  def explain(using Context) = ""
+}
+
 class NoReturnFromInlineable(owner: Symbol)(using Context)
   extends SyntaxMsg(NoReturnFromInlineableID) {
   def msg(using Context) = i"No explicit ${hl("return")} allowed from inlineable $owner"

tests/neg/17579.check

@@ -0,0 +1,26 @@
+-- [E199] Syntax Error: tests/neg/17579.scala:4:10 ---------------------------------------------------------------------
+4 |    final val v1 = 42 // error
+  |          ^^^
+  |          The final modifier is not allowed on local definitions
+-- [E199] Syntax Error: tests/neg/17579.scala:5:15 ---------------------------------------------------------------------
+5 |    final lazy val v2 = 42 // error
+  |               ^^^
+  |               The final modifier is not allowed on local definitions
+-- [E088] Syntax Error: tests/neg/17579.scala:6:10 ---------------------------------------------------------------------
+6 |    final private val v3 = 42 // error
+  |          ^^^^^^^
+  |          Expected start of definition
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E199] Syntax Error: tests/neg/17579.scala:7:10 ---------------------------------------------------------------------
+7 |    final def v4 = 42 // error
+  |          ^^^
+  |          The final modifier is not allowed on local definitions
+-- [E199] Syntax Error: tests/neg/17579.scala:8:10 ---------------------------------------------------------------------
+8 |    final var v5 = 42 // error
+  |          ^^^
+  |          The final modifier is not allowed on local definitions
+-- [E199] Syntax Error: tests/neg/17579.scala:9:10 ---------------------------------------------------------------------
+9 |    final type Foo = String // error
+  |          ^^^^
+  |          The final modifier is not allowed on local definitions

tests/neg/17579.scala

@@ -0,0 +1,10 @@
+class C:
+  final var v = 42 // ok
+  def f =
+    final val v1 = 42 // error
+    final lazy val v2 = 42 // error
+    final private val v3 = 42 // error
+    final def v4 = 42 // error
+    final var v5 = 42 // error
+    final type Foo = String // error
+    final given String = ???