Allow (...) around throws alternatives

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

@@ -1265,6 +1265,8 @@ object desugar {
    *  $throws[... $throws[A, E1] ... , En].
    */
   def throws(tpt: Tree, op: Ident, excepts: Tree)(using Context): AppliedTypeTree = excepts match
+    case Parens(excepts1) =>
+      throws(tpt, op, excepts1)
     case InfixOp(l, bar @ Ident(tpnme.raw.BAR), r) =>
       throws(throws(tpt, op, l), bar, r)
     case e =>

tests/pos/i13816.scala

@@ -0,0 +1,35 @@
+import language.experimental.saferExceptions
+
+class Ex1 extends Exception("Ex1")
+class Ex2 extends Exception("Ex2")
+
+def foo1(i: Int): Unit throws Ex1 throws Ex2 =
+   if i > 0 then throw new Ex1 else throw new Ex2
+
+def foo2(i: Int): Unit throws Ex1 | Ex2 =
+   if i > 0 then throw new Ex1 else throw new Ex2
+
+def foo2a(i: Int): Unit throws (Ex1 | Ex2) =
+   if i > 0 then throw new Ex1 else throw new Ex2
+
+def foo3(i: Int)(using CanThrow[Ex1], CanThrow[Ex2]) =
+   if i > 0 then throw new Ex1 else throw new Ex2
+
+def foo4(i: Int)(using CanThrow[Ex1])(using CanThrow[Ex2]) =
+   if i > 0 then throw new Ex1 else throw new Ex2
+
+//def foo5(i: Int)(using CanThrow[Ex1 & Ex2]) =      // does not work: no capability aggregation is supported
+//   if i > 0 then throw new Ex1 else throw new Ex2
+
+def test(): Unit =
+  try {
+    foo1(1)
+    foo2(1)
+    foo2a(1)
+    foo3(1)
+    foo4(1)
+    //foo5(1) // error
+  } catch {
+    case _: Ex1 =>
+    case _: Ex2 =>
+  }