Require implemented types to be traits

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -853,10 +853,13 @@ object desugar {
     else
       constr1 = addEvidenceParams(constr1, evidenceParams)
 
+    val mods =
+      if (isSimpleExtension) EmptyModifiers
+      else EmptyModifiers.withAddedMod(Mod.TraitImplementation())
     val icls =
       TypeDef(extName,
         cpy.Template(impl)(constr = constr1, parents = parents1, body = body1))
-        .withFlags(Implicit)
+        .withMods(mods.withFlags(Implicit))
     desugr.println(i"desugar $extended --> $icls")
     classDef(icls)
   }

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -142,6 +142,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     case class Enum() extends Mod(Flags.EmptyFlags)
 
     case class EnumCase() extends Mod(Flags.EmptyFlags)
+
+    case class TraitImplementation() extends Mod(Flags.EmptyFlags)
   }
 
   /** Modifiers and annotations for definitions

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -944,7 +944,8 @@ class Namer { typer: Typer =>
         if (cls.isRefinementClass) ptype
         else {
           val pt = checkClassType(ptype, parent.pos,
-              traitReq = parent ne parents.head, stablePrefixReq = true)
+              traitReq = (parent `ne` parents.head) || original.mods.hasMod[Mod.TraitImplementation],
+              stablePrefixReq = true)
           if (pt.derivesFrom(cls)) {
             val addendum = parent match {
               case Select(qual: Super, _) if ctx.scala2Mode =>

tests/neg/extensions.scala

@@ -30,7 +30,7 @@ object extensions {
 
   extend Circle2 extends HasArea {} // error: `implements` or `{` expected // error: `def` expected
 
-  extend Circle implements HasAreaClass {
+  extend Circle implements HasAreaClass { // error: class HasAreaClass is not a trait
     def area = this.radius * this.radius * math.Pi
   }