Reject capturing function type in polymorphic functions

Author: Linyxus

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1419,6 +1419,7 @@ object Parsers {
     private def getFunction(tree: Tree): Option[Function] = tree match {
       case Parens(tree1) => getFunction(tree1)
       case Block(Nil, tree1) => getFunction(tree1)
+      case Function(_, _: CapturesAndResult) => None
       case t: Function => Some(t)
       case _ => None
     }

tests/neg-custom-args/captures/i18518.scala

@@ -0,0 +1,5 @@
+import language.experimental.captureChecking
+type Foo1 = [R] -> (x: Unit) ->{} Unit  // error
+type Foo2 = [R] -> (x: Unit) ->{cap} Unit  // error
+type Foo3 = (c: Int^) -> [R] -> (x: Unit) ->{c} Unit  // error
+type Foo4 = (c: Int^) -> [R] -> (x0: Unit) -> (x: Unit) ->{c} Unit