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

Author: odersky

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

@@ -1810,6 +1810,10 @@ class Typer extends Namer
       // check value class constraints
       checkDerivedValueClass(cls, body1)
 
+      val effectiveOwner = cls.owner.skipWeakOwner
+      if !cls.isRefinementClass && effectiveOwner.is(Trait) && !effectiveOwner.derivesFrom(defn.ObjectClass)
+        ctx.error(i"class $cls cannot be defined in universal $effectiveOwner", 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/neg/i7748.scala

@@ -0,0 +1,11 @@
+trait A extends Any {
+    case class B() // error
+    val x = {
+      case class C() // error
+      1
+    }
+    def f = {
+      case class C() // ok
+      1
+    }
+}