Fix #7748: Don't allow classes to be defined in universal traits

Author: odersky

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

@@ -1810,6 +1810,9 @@ class Typer extends Namer
       // check value class constraints
       checkDerivedValueClass(cls, body1)
 
+      if !cls.owner.is(Package) && !cls.owner.derivesFrom(defn.ObjectClass)
+        ctx.error(i"classes cannot be defined in universal traits", cdef.sourcePos)
+
       // Temporarily set the typed class def as root tree so that we have at least some
       // information in the IDE in case we never reach `SetRootTree`.
       if (ctx.mode.is(Mode.Interactive) && ctx.settings.YretainTrees.value)

tests/pos/i7748.scala

@@ -0,0 +1,3 @@
+trait A extends Any {
+    case class B() // error
+}