From 9f1ff63c7fadcaaedbb5b57f00949d4aa5ff8934 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Doeraene?= <sjrdoeraene@gmail.com>
Date: Mon, 22 Jul 2024 11:22:49 +0200
Subject: [PATCH] =?UTF-8?q?Fix=20#20897:=20Make=20`Nothing=20=E2=8B=94=20N?=
 =?UTF-8?q?othing`=20and=20`Null=20=E2=8B=94=20Null`,=20as=20per=20spec.?=
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`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.)
---
 compiler/src/dotty/tools/dotc/core/TypeComparer.scala |  6 ++++--
 compiler/test/dotc/pos-test-pickling.blacklist        |  2 +-
 tests/pos/i20897.scala                                | 10 ++++++++++
 3 files changed, 15 insertions(+), 3 deletions(-)
 create mode 100644 tests/pos/i20897.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index c8e00686e62b..1f3806fa0cc4 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/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
diff --git a/compiler/test/dotc/pos-test-pickling.blacklist b/compiler/test/dotc/pos-test-pickling.blacklist
index d6f962176ecc..b68ac7fc3b6e 100644
--- a/compiler/test/dotc/pos-test-pickling.blacklist
+++ b/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
-
diff --git a/tests/pos/i20897.scala b/tests/pos/i20897.scala
new file mode 100644
index 000000000000..ecfac5b1615e
--- /dev/null
+++ b/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