Dealias in isSimpleThrowable check (#24744)

fixes: #24357

Author: hamzaremmal

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

@@ -71,7 +71,7 @@ class TryCatchPatterns extends MiniPhase {
     case _                                                               => isDefaultCase(cdef)
   }
 
-  private def isSimpleThrowable(tp: Type)(using Context): Boolean = tp.stripped match {
+  private def isSimpleThrowable(tp: Type)(using Context): Boolean = tp.strippedDealias match {
     case tp @ TypeRef(pre, _) =>
       (pre == NoPrefix || pre.typeSymbol.isStatic) && // Does not require outer class check
       !tp.symbol.is(Flags.Trait) && // Traits not supported by JVM

tests/run/i24357.scala

@@ -0,0 +1,15 @@
+class E1 extends Exception
+class E2 extends Exception
+
+type E1or2 = E1 | E2
+
+def caughtE1orE2(body: => Nothing): Boolean =
+  try body
+  catch
+    case ex: E1or2 => true
+    case _ => false
+
+@main def Test =
+    assert(caughtE1orE2(throw new E1 {}))
+    assert(caughtE1orE2(throw new E2 {}))
+    assert(!caughtE1orE2(throw new Exception {}))