Assuming content assist preferences "Fill method arguments and show guessed arguments" and "Place trailing closure arguments after closing parenthesis" are checked, completion of constructor with last param of type Closure produces invalid syntax.

class Foo {
  Foo(Number number, Closure closure) {
    closure()
  }
}

new Foo|

Content assist (Ctrl+Space) at | and selection of Foo() proposal results in replacement new Foo(number) { | }. This is mistaken for new anonymous inner type.