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.

llvm-svn: 312890

libcxx/include/functional

@@ -1597,9 +1597,11 @@ class _LIBCPP_TEMPLATE_VIS function<_Rp(_ArgTypes...)>
       return reinterpret_cast<__base*>(p);
     }
 
-    template <class _Fp, bool = !is_same<_Fp, function>::value &&
-                                __invokable<_Fp&, _ArgTypes...>::value>
-        struct __callable;
+    template <class _Fp, bool = __lazy_and<
+        integral_constant<bool, !is_same<__uncvref_t<_Fp>, function>::value>,
+        __invokable<_Fp&, _ArgTypes...>
+    >::value>
+    struct __callable;
     template <class _Fp>
         struct __callable<_Fp, true>
         {
@@ -1612,6 +1614,9 @@ class _LIBCPP_TEMPLATE_VIS function<_Rp(_ArgTypes...)>
         {
             static const bool value = false;
         };
+
+  template <class _Fp>
+  using _EnableIfCallable = typename enable_if<__callable<_Fp>::value>::type;
 public:
     typedef _Rp result_type;
 
@@ -1622,9 +1627,7 @@ public:
     function(nullptr_t) _NOEXCEPT : __f_(0) {}
     function(const function&);
     function(function&&) _NOEXCEPT;
-    template<class _Fp, class = typename enable_if<
-        __callable<_Fp>::value && !is_same<_Fp, function>::value
-    >::type>
+    template<class _Fp, class = _EnableIfCallable<_Fp>>
     function(_Fp);
 
 #if _LIBCPP_STD_VER <= 14
@@ -1638,21 +1641,15 @@ public:
       function(allocator_arg_t, const _Alloc&, const function&);
     template<class _Alloc>
       function(allocator_arg_t, const _Alloc&, function&&);
-    template<class _Fp, class _Alloc, class = typename enable_if<__callable<_Fp>::value>::type>
+    template<class _Fp, class _Alloc, class = _EnableIfCallable<_Fp>>
       function(allocator_arg_t, const _Alloc& __a, _Fp __f);
 #endif
 
     function& operator=(const function&);
     function& operator=(function&&) _NOEXCEPT;
     function& operator=(nullptr_t) _NOEXCEPT;
-    template<class _Fp>
-      typename enable_if
-      <
-        __callable<typename decay<_Fp>::type>::value &&
-        !is_same<typename remove_reference<_Fp>::type, function>::value,
-        function&
-      >::type
-      operator=(_Fp&&);
+    template<class _Fp, class = _EnableIfCallable<_Fp>>
+    function& operator=(_Fp&&);
 
     ~function();
 
@@ -1854,13 +1851,8 @@ function<_Rp(_ArgTypes...)>::operator=(nullptr_t) _NOEXCEPT
 }
 
 template<class _Rp, class ..._ArgTypes>
-template <class _Fp>
-typename enable_if
-<
-    function<_Rp(_ArgTypes...)>::template __callable<typename decay<_Fp>::type>::value &&
-    !is_same<typename remove_reference<_Fp>::type, function<_Rp(_ArgTypes...)>>::value,
-    function<_Rp(_ArgTypes...)>&
->::type
+template <class _Fp, class>
+function<_Rp(_ArgTypes...)>&
 function<_Rp(_ArgTypes...)>::operator=(_Fp&& __f)
 {
     function(_VSTD::forward<_Fp>(__f)).swap(*this);

libcxx/include/type_traits

@@ -4339,8 +4339,8 @@ struct __invokable_r
     using _Result = decltype(
         _VSTD::__invoke(_VSTD::declval<_Fp>(), _VSTD::declval<_Args>()...));
 
-    static const bool value =
-        conditional<
+    using type =
+        typename conditional<
             !is_same<_Result, __nat>::value,
             typename conditional<
                 is_void<_Ret>::value,
@@ -4348,7 +4348,8 @@ struct __invokable_r
                 is_convertible<_Result, _Ret>
             >::type,
             false_type
-        >::type::value;
+        >::type;
+    static const bool value = type::value;
 };
 
 template <class _Fp, class ..._Args>

libcxx/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);
+  }
+  {
+    const IncompleteReturnType Empty;
+    IncompleteReturnType X2 = Empty;
+    assert(!X2.fn);
+  }
+}
+
+int main() {
+  test_pr34298();
 }