Enforce rule that laziness is preserved when overriding.

Author: odersky

src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -131,7 +131,7 @@ class DottyBackendInterface()(implicit ctx: Context) extends BackendInterface{
 
   val hashMethodSym: Symbol = NoSymbol // used to dispatch ## on primitives to ScalaRuntime.hash. Should be implemented by a miniphase
   val externalEqualsNumNum: Symbol = defn.BoxesRunTimeModule.requiredMethod(nme.equalsNumNum)
-  lazy val externalEqualsNumChar: Symbol = ??? // ctx.requiredMethod(BoxesRunTimeTypeRef, nme.equalsNumChar) // this method is private
+  val externalEqualsNumChar: Symbol = NoSymbol // ctx.requiredMethod(BoxesRunTimeTypeRef, nme.equalsNumChar) // this method is private
   val externalEqualsNumObject: Symbol = defn.BoxesRunTimeModule.requiredMethod(nme.equalsNumObject)
   val externalEquals: Symbol = defn.BoxesRunTimeClass.info.decl(nme.equals_).suchThat(toDenot(_).info.firstParamTypes.size == 2).symbol
   val MaxFunctionArity: Int = Definitions.MaxFunctionArity

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

@@ -497,7 +497,7 @@ object Contexts {
   }
 
   @sharable object NoContext extends Context {
-    lazy val base = unsupported("base")
+    val base = null
     override val implicits: ContextualImplicits = new ContextualImplicits(Nil, null)(this)
   }
 

src/dotty/tools/dotc/typer/RefChecks.scala

@@ -211,7 +211,7 @@ object RefChecks {
         if (!(hasErrors && member.is(Synthetic) && member.is(Module))) {
           // suppress errors relating toi synthetic companion objects if other override
           // errors (e.g. relating to the companion class) have already been reported.
-          if (member.owner == clazz) ctx.error(fullmsg+", member = $member", member.pos)
+          if (member.owner == clazz) ctx.error(fullmsg, member.pos)
           else mixinOverrideErrors += new MixinOverrideError(member, fullmsg)
           hasErrors = true
         }
@@ -330,10 +330,12 @@ object RefChecks {
           "(this rule is designed to prevent ``accidental overrides'')")
       } else if (other.isStable && !member.isStable) { // (1.4)
         overrideError("needs to be a stable, immutable value")
-      } else if (member.is(Lazy) && !other.isRealMethod && !other.is(Deferred | Lazy)) {
-        overrideError("cannot override a concrete non-lazy value")
-      } else if (other.is(Lazy, butNot = Deferred) && !other.isRealMethod && !member.is(Lazy)) {
-        overrideError("must be declared lazy to override a concrete lazy value")
+      } else if (member.is(ModuleVal) && !other.isRealMethod && !other.is(Deferred | Lazy)) {
+        overrideError("may not override a concrete non-lazy value")
+      } else if (member.is(Lazy, butNot = Module) && !other.isRealMethod && !other.is(Lazy)) {
+        overrideError("may not override a non-lazy value")
+      } else if (other.is(Lazy) && !other.isRealMethod && !member.is(Lazy)) {
+        overrideError("must be declared lazy to override a lazy value")
       } else if (other.is(Deferred) && member.is(Macro) && member.extendedOverriddenSymbols.forall(_.is(Deferred))) { // (1.9)
         overrideError("cannot be used here - term macros cannot override abstract methods")
       } else if (other.is(Macro) && !member.is(Macro)) { // (1.10)

test/dotc/tests.scala

@@ -117,10 +117,10 @@ class tests extends CompilerTest {
   @Test def neg_autoTupling2 = compileFile(negDir, "autoTuplingTest", xerrors = 3)
   @Test def neg_companions = compileFile(negDir, "companions", xerrors = 1)
   @Test def neg_over = compileFile(negDir, "over", xerrors = 3)
-  @Test def neg_overrides = compileFile(negDir, "overrides", xerrors = 12)
+  @Test def neg_overrides = compileFile(negDir, "overrides", xerrors = 14)
   @Test def neg_overrideClass = compileFile(negDir, "overrideClass", List("-language:Scala2"), xerrors = 1)
   @Test def neg_i39 = compileFile(negDir, "i39", xerrors = 2)
-  @Test def neg_i50_volatile = compileFile(negDir, "i50-volatile", xerrors = 6)
+  @Test def neg_i50_volatile = compileFile(negDir, "i50-volatile", xerrors = 5)
   @Test def neg_zoo = compileFile(negDir, "zoo", xerrors = 12)
 
   val negTailcallDir = negDir + "tailcall/"

tests/neg/overrides.scala

@@ -3,10 +3,16 @@ class Foo {
   type B >: Int <: Int
   def get: A = 42
 }
-class Bar extends Foo {
+trait T {
+  lazy val x: Int
+  val y: Int
+}
+class Bar extends Foo with T {
   override type A = Any   // error
   type B >: String <: Any  // error
   override def get: A = "bar"
+  val x = 2  // error
+  lazy val y = 3 // error
 }
 object Test {
   def main(args: Array[String]): Unit = {

tests/pos-scala2/pos-special/i871.scala

@@ -0,0 +1,4 @@
+trait Message {
+  def first(x: Int)
+  def second
+}

tests/run/t429.scala

@@ -1,7 +1,7 @@
 object Test {
   abstract class A {
     Console.print("A");
-    val x: Int;
+    lazy val x: Int;
     val y: Int = {Console.print("y"); x + 1}
   }
   class B extends A {