Fix handling of Nothing in OrType atoms

Author: dwijnand

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

@@ -3483,9 +3483,11 @@ object Types {
     private var myWidened: Type = _
 
     private def computeAtoms()(using Context): Atoms =
-      if tp1.hasClassSymbol(defn.NothingClass) then tp2.atoms
-      else if tp2.hasClassSymbol(defn.NothingClass) then tp1.atoms
-      else tp1.atoms | tp2.atoms
+      val tp1n = tp1.normalized
+      val tp2n = tp2.normalized
+      if tp1n.hasClassSymbol(defn.NothingClass) then tp2.atoms
+      else if tp2n.hasClassSymbol(defn.NothingClass) then tp1.atoms
+      else tp1n.atoms | tp2n.atoms
 
     private def computeWidenSingletons()(using Context): Type =
       val tp1w = tp1.widenSingletons

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

@@ -52,6 +52,7 @@ i15155.scala
 i15827.scala
 i17149.scala
 tuple-fold.scala
+mt-redux-norm.perspective.scala
 
 # Opaque type
 i5720.scala

tests/pos/mt-redux-norm.perspective.scala

@@ -0,0 +1,25 @@
+// while making MT post-redux consistent in its normalisation/simplification
+// one version of the change broke a line of the perspective community project in CI
+// this is a minimisation of the failure
+
+import scala.compiletime._, scala.deriving._
+
+transparent inline def foo(using m: Mirror): Unit =
+  constValueTuple[m.MirroredElemLabels].toList.toSet // was:
+//-- [E057] Type Mismatch Error: cat.scala:8:14 ----------------------------------
+//8 |def test = foo(using summon[Mirror.Of[Cat]])
+//  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+//  |Type argument ("name" : String) | Nothing does not conform to lower bound m$proxy1.MirroredElemLabels match {
+//  |  case EmptyTuple => Nothing
+//  |  case h *: t => h | Tuple.Fold[t, Nothing, [x, y] =>> x | y]
+//  |}
+//  |-----------------------------------------------------------------------------
+//  |Inline stack trace
+//  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+//  |This location contains code that was inlined from cat.scala:4
+//4 |  constValueTuple[m.MirroredElemLabels].toList.toSet
+//  |                                               ^
+
+case class Cat(name: String)
+
+def test = foo(using summon[Mirror.Of[Cat]])