diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index ab0e9b3ed9d4..6020df3d9e5b 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1575,6 +1575,17 @@ object SymDenotations {
           (symbol eq defn.NothingClass) ||
             (symbol eq defn.NullClass) && (base ne defn.NothingClass))
 
+    /** Is it possible that a class inherits both `this` and `that`?
+     *
+     *  @note The test is based on single-class inheritance and the closed
+     *        hierarchy of final classes.
+     *
+     *  @return The result may contain false positives, but never false negatives.
+     */
+    final def mayHaveCommonChild(that: ClassSymbol)(implicit ctx: Context): Boolean =
+      !this.is(Final) && !that.is(Final) && (this.is(Trait) || that.is(Trait)) ||
+        this.derivesFrom(that) || that.derivesFrom(this.symbol)
+
     final override def typeParamCreationFlags: FlagSet = ClassTypeParamCreationFlags
 
     /** Hook to do a pre-enter test. Overridden in PackageDenotation */
diff --git a/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala b/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
index dbd781357c23..b14024e933c5 100644
--- a/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
@@ -132,6 +132,7 @@ object TypeTestsCasts {
             recur(tp1, P) && recur(tp2, P)
           case _ =>
             // first try withou striping type parameters for performance
+            X.classSymbol.exists && P.classSymbol.exists && !X.classSymbol.asClass.mayHaveCommonChild(P.classSymbol.asClass) ||
             isClassDetermined(X, tpe)(ctx.fresh.setNewTyperState()) ||
             isClassDetermined(stripTypeParam(X), tpe)(ctx.fresh.setNewTyperState())
         }
diff --git a/tests/pos-special/fatal-warnings/i5970.scala b/tests/pos-special/fatal-warnings/i5970.scala
new file mode 100644
index 000000000000..51adb8cd0535
--- /dev/null
+++ b/tests/pos-special/fatal-warnings/i5970.scala
@@ -0,0 +1,12 @@
+object Test extends App {
+  case class Foo[T](t: T)
+
+  def foo[T](ft: Unit|Foo[T]): T = {
+    ft match {
+      case Foo(t) => t
+      case () => ???
+    }
+  }
+
+  foo(Foo(23))
+}