Correctly handle abstract type in isCheckDefinitelyFalse

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -153,11 +153,14 @@ object TypeTestsCasts {
             val xs = xSpace.decompose.map(_.tp)
             xs.forall(x => isCheckDefinitelyFalse(x, p))
           else
-            val xClass = effectiveClass(x.widen)
-            val pClass = effectiveClass(p.widen)
+            if x.typeSymbol.isClass && p.typeSymbol.isClass then
+              val xClass = effectiveClass(x.widen)
+              val pClass = effectiveClass(p.widen)
 
-            !xClass.derivesFrom(pClass)
-            && (xClass.is(Final) || pClass.is(Final) || !xClass.is(Trait) && !pClass.is(Trait))
+              !xClass.derivesFrom(pClass)
+              && (xClass.is(Final) || pClass.is(Final) || !xClass.is(Trait) && !pClass.is(Trait))
+            else
+              false
     }
 
     def recur(X: Type, P: Type): String = (X <:< P) ||| (P.dealias match {

tests/neg-custom-args/isInstanceOf/i5826.scala

@@ -1,6 +1,6 @@
 class Foo {
-  def test[A]: List[Int] | A => Int = {
-    case ls: List[Int] => ls.head       // ok, List decomposes to Some and None
+  def test[A]: (List[Int] | A) => Int = {
+    case ls: List[Int] => ls.head       // error, A = List[String]
     case _ => 0
   }
 
@@ -17,4 +17,9 @@ class Foo {
     case ls: A[X] => 4                 // error
     case _ => 0
   }
+
+  def test4[A](x: List[Int] | (A => Int)) = x match {
+    case ls: List[Int] => ls.head       // ok, List decomposes to Some and None
+    case _ => 0
+  }
 }