Revert "Fix PR34298 - Allow std::function with an incomplete return type."

This reverts commit r312890 because the test case fails to compile for
older versions of Clang that reject initializing a const object without
a user defined constructor.

Since this patch should go into 5.0.1, I want to keep it an atomic change,
and will re-commit it with a fixed test case.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@312891 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/F_incomplete.pass.cpp

@@ -16,7 +16,6 @@
 // Allow incomplete argument types in the __is_callable check
 
 #include <functional>
-#include <cassert>
 
 struct X{
     typedef std::function<void(X&)> callback_type;
@@ -25,40 +24,6 @@ private:
     callback_type _cb;
 };
 
-struct IncompleteReturnType {
-  std::function<IncompleteReturnType ()> fn;
-};
-
-
-int called = 0;
-IncompleteReturnType test_fn() {
-  ++called;
-  IncompleteReturnType I;
-  return I;
-}
-
-// See llvm.org/PR34298
-void test_pr34298()
+int main()
 {
-  static_assert(std::is_copy_constructible<IncompleteReturnType>::value, "");
-  static_assert(std::is_copy_assignable<IncompleteReturnType>::value, "");
-  {
-    IncompleteReturnType X;
-    X.fn = test_fn;
-    const IncompleteReturnType& CX = X;
-    IncompleteReturnType X2 = CX;
-    assert(X2.fn);
-    assert(called == 0);
-    X2.fn();
-    assert(called == 1);
-  }
-  {
-    const IncompleteReturnType Empty;
-    IncompleteReturnType X2 = Empty;
-    assert(!X2.fn);
-  }
-}
-
-int main() {
-  test_pr34298();
 }