Fix provablyDisjoint handling enum constants with mixins (scala#21876)

Fixes scala#21860

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

@@ -3196,9 +3196,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       cls.is(Sealed) && !cls.hasAnonymousChild
 
     def decompose(cls: Symbol): List[Symbol] =
-      cls.children.map { child =>
-        if child.isTerm then child.info.classSymbol
-        else child
+      cls.children.flatMap { child =>
+        if child.isTerm then
+          child.info.classSymbols // allow enum vals to be decomposed to their enum class (then filtered out) and any mixins
+        else child :: Nil
       }.filter(child => child.exists && child != cls)
 
     def eitherDerivesFromOther(cls1: Symbol, cls2: Symbol): Boolean =

tests/warn/i21860.scala

@@ -0,0 +1,16 @@
+trait Figure
+sealed trait Corners { self: Figure => }
+
+enum Shape extends Figure:
+  case Triangle extends Shape with Corners
+  case Square   extends Shape with Corners
+  case Circle   extends Shape
+  case Ellipsis extends Shape
+
+def hasCorners(s: Shape): Boolean = s match
+  case hasCorners: Corners => true //  <--- reported as `Unreachable case`
+  case _                   => false
+
+class Test:
+  def test(): Unit =
+    println(hasCorners(Shape.Circle))

tests/warn/i21860.unenum.scala

@@ -0,0 +1,17 @@
+trait Figure
+sealed trait Corners { self: Figure => }
+
+sealed abstract class Shape extends Figure
+object Shape:
+  case object Triange  extends Shape with Corners
+  case object Square   extends Shape with Corners
+  case object Circle   extends Shape
+  case object Ellipsis extends Shape
+
+def hasCorners(s: Shape): Boolean = s match
+  case hasCorners: Corners => true //  <--- reported as `Unreachable case`
+  case _                   => false
+
+class Test:
+  def test(): Unit =
+    println(hasCorners(Shape.Circle))