Add thrown exceptions to generic java signature

Author: Duhemm

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

@@ -1035,11 +1035,23 @@ object Erasure {
     else None
   }
 
-  // TODO: Port from scalac
-  private object ThrownException {
-    def unapply(ann: Annotation): Option[Type] = None
+  /** Extracts the type of the thrown exception from an AnnotationInfo.
+    *
+    * Supports both “old-style” `@throws(classOf[Exception])`
+    * as well as “new-style” `@throws[Exception]("cause")` annotations.
+    */
+  object ThrownException {
+    def unapply(ann: Annotation)(implicit ctx: Context): Option[Type] = {
+      ann.tree match {
+        case Apply(TypeApply(fun, List(tpe)), _) if tpe.isType && fun.symbol.owner == defn.ThrowsAnnot && fun.symbol.isConstructor =>
+          Some(tpe.typeOpt)
+        case _ =>
+          None
+      }
+    }
   }
 
+
   class UnknownSig extends Exception
 
   private def needsJavaSig(tp: Type, throwsArgs: List[Type])(implicit ctx: Context): Boolean = !ctx.settings.YnoGenericSig.value && {