Fix #1688: don't allow override of synthetic or primitive classes

src/dotty/tools/dotc/Compiler.scala

@@ -50,6 +50,7 @@ class Compiler {
            new CheckReentrant),     // Internal use only: Check that compiled program has no data races involving global vars
       List(new RefChecks,           // Various checks mostly related to abstract members and overriding
            new CheckStatic,         // Check restrictions that apply to @static members
+           new CheckIllegalOverrides, // Check restrictions that apply to phantom types and value classes
            new ElimRepeated,        // Rewrite vararg parameters and arguments
            new NormalizeFlags,      // Rewrite some definition flags
            new ExtensionMethods,    // Expand methods of value classes with extension methods

src/dotty/tools/dotc/core/Definitions.scala

@@ -633,9 +633,9 @@ class Definitions {
     name.startsWith(prefix) && name.drop(prefix.length).forall(_.isDigit)
   }
 
-  def isBottomClass(cls: Symbol) = 
+  def isBottomClass(cls: Symbol) =
     cls == NothingClass || cls == NullClass
-  def isBottomType(tp: Type) = 
+  def isBottomType(tp: Type) =
     tp.derivesFrom(NothingClass) || tp.derivesFrom(NullClass)
 
   def isFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.Function)
@@ -772,7 +772,8 @@ class Definitions {
     SingletonClass,
     EqualsPatternClass,
     EmptyPackageVal,
-    OpsPackageClass)
+    OpsPackageClass
+  )
 
     /** Lists core methods that don't have underlying bytecode, but are synthesized on-the-fly in every reflection universe */
     lazy val syntheticCoreMethods = AnyMethods ++ ObjectMethods ++ List(String_+, throwMethod)
@@ -785,8 +786,8 @@ class Definitions {
     if (!_isInitialized) {
       // force initialization of every symbol that is synthesized or hijacked by the compiler
       val forced = syntheticCoreClasses ++ syntheticCoreMethods ++ ScalaValueClasses()
-      
-      // Enter all symbols from the scalaShadowing package in the scala package 
+
+      // Enter all symbols from the scalaShadowing package in the scala package
       for (m <- ScalaShadowingPackageClass.info.decls)
         ScalaPackageClass.enter(m)
 

src/dotty/tools/dotc/transform/CheckIllegalOverrides.scala

@@ -0,0 +1,41 @@
+package dotty.tools.dotc
+package transform
+
+import TreeTransforms._
+import core._
+import Contexts._, Definitions._, Decorators._
+
+/** Checks if the user is trying to illegally override synthetic classes,
+  * primitives or their boxed equivalent
+  */
+class CheckIllegalOverrides extends MiniPhaseTransform {
+  import ast.tpd._
+
+  override def phaseName = "checkPhantom"
+
+  override def transformTypeDef(tree: TypeDef)(implicit ctx: Context, info: TransformerInfo) = {
+    val defn = ctx.definitions
+    val tpe = tree.tpe
+    val fullName = tree.symbol.showFullName
+
+    val syntheticClasses = defn.syntheticCoreClasses.map(_.showFullName)
+    val primitives = defn.ScalaValueClasses().map(_.showFullName)
+    val boxedPrimitives = defn.ScalaBoxedClasses().map(_.showFullName)
+
+    val illegalOverride =
+      if (syntheticClasses.contains(fullName))
+        Some("synthetic")
+      else if (primitives.contains(fullName))
+        Some("primitive")
+      else if (boxedPrimitives.contains(fullName))
+        Some("boxed primitive")
+      else
+        None
+
+    illegalOverride.foreach { overriden =>
+      ctx.error(i"Cannot define symbol overriding $overriden class `$tpe`", tree.pos)
+    }
+
+    tree
+  }
+}

tests/neg/i1688.scala

@@ -0,0 +1,5 @@
+package scala
+
+sealed trait Null // error
+
+trait Char // error

tests/neg/i1688b.scala

@@ -0,0 +1,3 @@
+package java.lang
+
+trait Byte // error