Handle NoType in TypeComparer.disjointnessBoundary

Closes scala#20265

Author: wjoel

tests/neg/i20265.check

@@ -0,0 +1,28 @@
+-- [E172] Type Error: tests/neg/i20265.scala:22:95 ---------------------------------------------------------------------
+22 |  println(summon[((String --> Unit) * (String --> Unit)) =:= Hinze[(String + String) --> Unit]]) // error
+   |                                                                                               ^
+   |                    Cannot prove that (String --> Unit) * (String --> Unit) =:= Hinze[String + String --> Unit].
+   |
+   |                    Note: a match type could not be fully reduced:
+   |
+   |                      trying to reduce  Hinze[String + String --> Unit]
+   |                      failed since selector (String + String --> Unit)#unfix
+   |                      does not match  case k1 + k2 --> v => Hinze[k1 --> v] * Hinze[k2 --> v]
+   |                      and cannot be shown to be disjoint from it either.
+   |                      Therefore, reduction cannot advance to the remaining case
+   |
+   |                        case k1 * k2 --> v => k1 --> Hinze[k2 --> v]
+-- [E172] Type Error: tests/neg/i20265.scala:23:66 ---------------------------------------------------------------------
+23 |  println(summon[String =:= Hinze[Fix[Lambda[String]#L] --> Unit]]) // error
+   |                                                                  ^
+   |                                       Cannot prove that String =:= Hinze[Fix[Lambda[String]#L] --> Unit].
+   |
+   |                                       Note: a match type could not be fully reduced:
+   |
+   |                                         trying to reduce  Hinze[Fix[Lambda[String]#L] --> Unit]
+   |                                         failed since selector (Fix[Lambda[String]#L] --> Unit)#unfix
+   |                                         does not match  case k1 + k2 --> v => Hinze[k1 --> v] * Hinze[k2 --> v]
+   |                                         and cannot be shown to be disjoint from it either.
+   |                                         Therefore, reduction cannot advance to the remaining case
+   |
+   |                                           case k1 * k2 --> v => k1 --> Hinze[k2 --> v]

tests/neg/i20265.scala

@@ -0,0 +1,23 @@
+//> using options -source:3.3
+
+trait Poly
+trait -->[X, Y] extends Poly
+trait +[X, Y] extends Poly
+trait *[X, Y] extends Poly
+
+type Hinze[X <: Fix[?]] = X#unfix match
+  case (k1 + k2) --> v => Hinze[(k1 --> v)] * Hinze[(k2 --> v)]
+  case (k1 * k2) --> v => k1 --> Hinze[(k2 --> v)]
+
+trait Lambda[V]:
+  type Abs[X] = V * X
+  type App[X] = X * X
+  type L[X] = V + Abs[X] + App[X]
+
+trait Fix[F[X]]:
+  type unfix = F[Fix[F]]
+
+@main
+def m =
+  println(summon[((String --> Unit) * (String --> Unit)) =:= Hinze[(String + String) --> Unit]]) // error
+  println(summon[String =:= Hinze[Fix[Lambda[String]#L] --> Unit]]) // error