Fix #9788: Skip *: when computing erased lub

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -331,8 +331,10 @@ object TypeErasure {
             val candidates = takeUntil(tp2superclasses)(!_.is(Trait))
 
             // Candidates st "no other common superclass or trait derives from S"
+            // Also, drop `PairClass` since it is not valid after erasue
             val minimums = candidates.filter { cand =>
-              candidates.forall(x => !x.derivesFrom(cand) || x.eq(cand))
+              cand != defn.PairClass
+              && candidates.forall(x => !x.derivesFrom(cand) || x.eq(cand))
             }
 
             // Pick the last minimum to prioritise classes over traits

tests/pos/i9788.scala

@@ -0,0 +1 @@
+case class A(x: (Unit, Unit) | (Unit, Unit, Unit))