Fix #6890: Disallow MT on the RHS of type aliases

Author: OlivierBlanvillain

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

@@ -1128,7 +1128,7 @@ object desugar {
     case TypeDef(_, rhs) =>
       def rhsOK(tree: Tree): Boolean = tree match {
         case bounds: TypeBoundsTree => !bounds.alias.isEmpty
-        case _: Template => false
+        case _: Template | _: MatchTypeTree => false
         case LambdaTypeTree(_, body) => rhsOK(body)
         case _ => true
       }

tests/neg/6890.scala

@@ -0,0 +1,3 @@
+object Test {
+  opaque type A[T] = T match { case Int => Int } // error: Modifier `opaque` is not allowed for this definition
+}