Fix scala#20897: Make Nothing ⋔ Nothing and Null ⋔ Null, as per spec.

`derivesFrom` normally returns `false` when the receiver is
`Nothing` or `Null`. However, it returns `true` if the right-hand-
side happens to be exactly the same class. For the purpose of
computing `provablyDisjoint`, we have to explicitly exclude those.

(Note that the spec text per se only explicitly specifies
`Nothing ⋔ Nothing`, not `Null ⋔ Null`, but that's because the spec
text punts on the `Null` issue. The intent of `provablyDisjoint`
is that there is no *non-null* value belonging to both types.)

Author: sjrd

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

@@ -3187,9 +3187,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         else child
       }.filter(child => child.exists && child != cls)
 
-    // TODO? Special-case for Nothing and Null? We probably need Nothing/Null disjoint from Nothing/Null
     def eitherDerivesFromOther(cls1: Symbol, cls2: Symbol): Boolean =
-      cls1.derivesFrom(cls2) || cls2.derivesFrom(cls1)
+      if cls1 eq cls2 then
+        cls1 != defn.NothingClass && cls1 != defn.NullClass
+      else
+        cls1.derivesFrom(cls2) || cls2.derivesFrom(cls1)
 
     def smallestNonTraitBase(cls: Symbol): Symbol =
       cls.asClass.baseClasses.find(!_.is(Trait)).get

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

@@ -67,6 +67,7 @@ mt-redux-norm.perspective.scala
 i18211.scala
 10867.scala
 named-tuples1.scala
+i20897.scala
 
 # Opaque type
 i5720.scala
@@ -134,4 +135,3 @@ parsercombinators-new-syntax.scala
 hylolib-deferred-given
 hylolib-cb
 hylolib
-

tests/pos/i20897.scala

@@ -0,0 +1,10 @@
+object Test:
+  type Disj[A, B] =
+    A match
+      case B => true
+      case _ => false
+
+  def f(a: Disj[1 | Nothing, 2 | Nothing]): Unit = ()
+
+  val t = f(false)
+end Test