Disallow overriding opaque type aliases

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

@@ -305,7 +305,7 @@ object RefChecks {
       def info = self.memberInfo(sym1)
       val infoStr =
         if (sym1.isAliasType) i", which equals ${info.bounds.hi}"
-        else if (sym1.isAbstractOrParamType) i" with bounds$info"
+        else if (sym1.isAbstractOrParamType && info != TypeBounds.empty) i" with bounds$info"
         else if (sym1.is(Module)) ""
         else if (sym1.isTerm) i" of type $info"
         else ""
@@ -430,6 +430,10 @@ object RefChecks {
         // direct overrides were already checked on completion (see Checking.chckWellFormed)
         // the test here catches indirect overriddes between two inherited base types.
         overrideError("cannot be used here - class definitions cannot be overridden")
+      else if (other.isOpaqueAlias)
+        // direct overrides were already checked on completion (see Checking.chckWellFormed)
+        // the test here catches indirect overriddes between two inherited base types.
+        overrideError("cannot be used here - opaque type aliases cannot be overridden")
       else if (!other.is(Deferred) && member.isClass)
         overrideError("cannot be used here - classes can only override abstract types")
       else if other.isEffectivelyFinal then // (1.2)

tests/neg/i14659.scala

@@ -0,0 +1,13 @@
+trait Foo {
+  opaque type Out = Int
+  def out1: Out
+  def out2: Out = out1
+}
+
+object Bar extends Foo {
+  override opaque type Out = String // error
+  override def out1 = "abc"
+}
+
+@main def run() =
+  val x = Bar.out2