Fix #8662: Fix desugaring of try-catches that are partial functions

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1612,6 +1612,17 @@ object desugar {
         EmptyTree
     }
 
+    def makeTryCase(handlerFun: Tree): CaseDef =
+      val excId = Ident(nme.DEFAULT_EXCEPTION_NAME)
+      val tmpName = UniqueName.fresh()
+      val tmpId = Ident(tmpName)
+      val init = ValDef(tmpName, TypeTree(), handlerFun)
+      val test = If(
+        Apply(Select(tmpId, nme.isDefinedAt), excId),
+        Apply(Select(tmpId, nme.apply), excId),
+        Throw(excId))
+      CaseDef(excId, EmptyTree, Block(init :: Nil, test))
+
     // begin desugar
 
     // Special case for `Parens` desugaring: unlike all the desugarings below,
@@ -1677,14 +1688,10 @@ object desugar {
         val pats1 = if (tpt.isEmpty) pats else pats map (Typed(_, tpt))
         flatTree(pats1 map (makePatDef(tree, mods, _, rhs)))
       case ParsedTry(body, handler, finalizer) =>
-        handler match {
+        handler match
           case Match(EmptyTree, cases) => Try(body, cases, finalizer)
           case EmptyTree => Try(body, Nil, finalizer)
-          case _ =>
-            Try(body,
-              List(CaseDef(Ident(nme.DEFAULT_EXCEPTION_NAME), EmptyTree, Apply(handler, Ident(nme.DEFAULT_EXCEPTION_NAME)))),
-              finalizer)
-        }
+          case _ => Try(body, makeTryCase(handler) :: Nil, finalizer)
     }
     desugared.withSpan(tree.span)
   }

tests/run/i8662.scala

@@ -0,0 +1,27 @@
+object Main {
+  class EThrow extends Exception
+  class ECatch extends Exception
+
+  def ok(): Unit =
+    try throw new EThrow
+    catch { case _: ECatch => }
+
+  def notOk(): Unit = {
+    val pf: PartialFunction[Throwable, Unit] = {
+      case e: ECatch =>
+    }
+    try throw new EThrow
+    catch pf
+  }
+
+  def test(f: => Unit): Unit =
+    try f catch {
+      case _: EThrow    => println("OK")
+      // case e: Throwable => println("! expect an EThrow but got " + e.getClass)
+    }
+
+  def main(args: Array[String]): Unit = {
+    test { ok() }
+    test { notOk() }
+  }
+}