Fix scala#6687: handle gadtBounds in MT reduction

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

@@ -2388,7 +2388,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case _ => false
     }
 
-
   /** Are `tp1` and `tp2` provablyDisjoint types?
    *
    *  `true` implies that we found a proof; uncertainty defaults to `false`.
@@ -2507,6 +2506,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case (_, tp2: AndType) =>
         !(tp2 <:< tp1)
         && (provablyDisjoint(tp1, tp2.tp2) || provablyDisjoint(tp1, tp2.tp1))
+      case (tp1: NamedType, _) if gadtBounds(tp1.symbol) != null =>
+        provablyDisjoint(gadtBounds(tp1.symbol).hi, tp2) || provablyDisjoint(tp1.superType, tp2)
+      case (_, tp2: NamedType) if gadtBounds(tp2.symbol) != null =>
+        provablyDisjoint(tp1, gadtBounds(tp2.symbol).hi) || provablyDisjoint(tp1, tp2.superType)
       case (tp1: TypeProxy, tp2: TypeProxy) =>
         provablyDisjoint(tp1.superType, tp2) || provablyDisjoint(tp1, tp2.superType)
       case (tp1: TypeProxy, _) =>

compiler/test/dotc/pos-test-pickling.blacklist

@@ -30,6 +30,7 @@ i7087.scala
 i7868.scala
 i7872.scala
 6709.scala
+6687.scala
 
 # Opaque type
 i5720.scala

tests/pos/6687.scala

@@ -0,0 +1,11 @@
+type T[X] = X match {
+  case String => Int
+  case Int => String
+}
+
+class Box[X](x: X)
+
+def f[X](x: Box[X]): T[X] = x match {
+  case x: Box[Int]    => ""
+  case x: Box[String] => 1
+}