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

This patch fixes llvm.org/PR34298. Previously libc++ incorrectly evaluated
the __invokable trait via the converting constructor `function(Tp)` [with Tp = std::function]
whenever the copy constructor or copy assignment operator
was required. This patch further constrains that constructor to short
circut before evaluating the troublesome SFINAE when `Tp` matches
std::function.

The original patch is from Alex Lorenz.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@312892 91177308-0d34-0410-b5e6-96231b3b80d8

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

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