Add a specific error message for local final defs (#20557)

Fixes #17579.

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

@@ -4560,6 +4560,12 @@ 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
+
+        // See test 17579. We allow `final` on `given` because these can be
+        // translated to class definitions, for which `final` is allowed but
+        // redundant--there is a seperate warning for this.
+        if isDclIntro && in.token != GIVEN then syntaxError(FinalLocalDef())
+
         tmplDef(start, mods)
       else
         defOrDcl(start, mods)

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

@@ -213,6 +213,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case InlinedAnonClassWarningID // errorNumber: 197
   case UnusedSymbolID // errorNumber: 198
   case TailrecNestedCallID //errorNumber: 199
+  case FinalLocalDefID // errorNumber: 200
 
   def errorNumber = ordinal - 1
 

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

@@ -1838,7 +1838,7 @@ class WrongNumberOfParameters(tree: untpd.Tree, foundCount: Int, pt: Type, expec
 
 class DuplicatePrivateProtectedQualifier()(using Context)
   extends SyntaxMsg(DuplicatePrivateProtectedQualifierID) {
-  def msg(using Context) = "Duplicate private/protected qualifier"
+  def msg(using Context) = "Duplicate private/protected modifier"
   def explain(using Context) =
     i"It is not allowed to combine `private` and `protected` modifiers even if they are qualified to different scopes"
 }
@@ -1847,7 +1847,13 @@ class ExpectedStartOfTopLevelDefinition()(using Context)
   extends SyntaxMsg(ExpectedStartOfTopLevelDefinitionID) {
   def msg(using Context) = "Expected start of definition"
   def explain(using Context) =
-    i"You have to provide either ${hl("class")}, ${hl("trait")}, ${hl("object")}, or ${hl("enum")} definitions after qualifiers"
+    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)

tests/neg/17579.check

@@ -0,0 +1,30 @@
+-- [E200] Syntax Error: tests/neg/17579.scala:5:10 ---------------------------------------------------------------------
+5 |    final val v1 = 42 // error: final modifier is not allowed on local definitions
+  |          ^^^
+  |          The final modifier is not allowed on local definitions
+-- [E200] Syntax Error: tests/neg/17579.scala:6:15 ---------------------------------------------------------------------
+6 |    final lazy val v2 = 42 // error:  final modifier is not allowed on local definitions
+  |               ^^^
+  |               The final modifier is not allowed on local definitions
+-- [E200] Syntax Error: tests/neg/17579.scala:7:10 ---------------------------------------------------------------------
+7 |    final def v4 = 42 // error: final modifier is not allowed on local definitions
+  |          ^^^
+  |          The final modifier is not allowed on local definitions
+-- [E200] Syntax Error: tests/neg/17579.scala:8:10 ---------------------------------------------------------------------
+8 |    final var v5 = 42 // error: final modifier is not allowed on local definitions
+  |          ^^^
+  |          The final modifier is not allowed on local definitions
+-- [E200] Syntax Error: tests/neg/17579.scala:9:10 ---------------------------------------------------------------------
+9 |    final type Foo = String // error: final modifier is not allowed on local definitions
+  |          ^^^^
+  |          The final modifier is not allowed on local definitions
+-- [E088] Syntax Error: tests/neg/17579.scala:14:10 --------------------------------------------------------------------
+14 |    final private val v3 = 42 // error: expected start of definition
+   |          ^^^^^^^
+   |          Expected start of definition
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E147] Syntax Warning: tests/neg/17579.scala:19:6 -------------------------------------------------------------------
+19 |      final given Object with {} // warning: modifier `final` is redundant for this definition
+   |      ^^^^^
+   |      Modifier final is redundant for this definition

tests/neg/17579.scala

@@ -0,0 +1,26 @@
+class C:
+  final var v = 42 // ok
+
+  def f =
+    final val v1 = 42 // error: final modifier is not allowed on local definitions
+    final lazy val v2 = 42 // error:  final modifier is not allowed on local definitions
+    final def v4 = 42 // error: final modifier is not allowed on local definitions
+    final var v5 = 42 // error: final modifier is not allowed on local definitions
+    final type Foo = String // error: final modifier is not allowed on local definitions
+
+    // We get a different error message here because `private` is also not a
+    // local modifier token. In the future, we could always parse all tokens and
+    // then flag those that are not legal at this point.
+    final private val v3 = 42 // error: expected start of definition
+
+    {
+      // No error in this case, because the `given` is translated to a class
+      // definition, for which `final` is redundant but not illegal.
+      final given Object with {} // warning: modifier `final` is redundant for this definition
+    }
+
+    {
+      // Also no error in this case, because we can't easily distinguish it from
+      // the previous case.
+      final given Object = new Object {}
+    }