[libc++] Fix PR20084 - std::is_function<void() const> failed.

Summary:
This patch introduces some black magic to detect const and volatile qualified function types such as `void () const`.

The patch works in the following way:

We first rule out any type that satisfies on of the following. These restrictions are important so that the test below works properly.
* `is_class<_Tp>::value`
* `is_union<_Tp>::value`
* `is_void<_Tp>::value`
* `is_reference<_Tp>::value`
* `__is_nullptr_t<_Tp>::value`


If none of the above is true we perform overload resolution on `__source<_Tp>(0)` to determine the return type.
*  If `_Tp&` is well-formed we select `_Tp& __source(int)`. `_Tp&` is only ill formed for cv void types and cv/ref qualified function types.
* Otherwise we select `__dummy_type __source(...)`. Since we know `_Tp` cannot be void then it must be a function type.


let `R` be the returned from `__source<_Tp>(0)`. 
We perform overload resolution on `__test<_Tp>(R)`.
* If `R` is `__dummy_type` we call `true_type __test(__dummy_type)`.
* if `R` is `_Tp&` and `_Tp&` decays to `_Tp*` we call `true_type __test(_Tp*)`.  Only references to function types decay to a pointer of the same type.
* In all other cases we call `false_type __test(...)`. 

`__source<_Tp>(0)` will try and form `_Tp&`  in the return type. if `_Tp&` is not well formed the return type of `__source<_Tp>(0)` will be dummy type. `_Tp&` is only ill-formed for cv/ref qualified function types (and void which is dealt with elsewhere).


This fixes PR20084 - http://llvm.org/bugs/show_bug.cgi?id=20084

Reviewers: rsmith, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7573

llvm-svn: 229696

2 files changed, 54 insertions, 15 deletions

diff --git a/libcxx/include/type_traits b/libcxx/include/type_traits
index c44cae63908..52fb5902485 100644
--- a/libcxx/include/type_traits
+++ b/libcxx/include/type_traits
@@ -430,9 +430,12 @@ template <class _Tp>            struct _LIBCPP_TYPE_VIS_ONLY is_same<_Tp, _Tp> :
 
 namespace __libcpp_is_function_imp
 {
+struct __dummy_type {};
 template <class _Tp> char  __test(_Tp*);
+template <class _Tp> char __test(__dummy_type);
 template <class _Tp> __two __test(...);
-template <class _Tp> _Tp&  __source();
+template <class _Tp> _Tp&  __source(int);
+template <class _Tp> __dummy_type __source(...);
 }
 
 template <class _Tp, bool = is_class<_Tp>::value ||
@@ -441,7 +444,7 @@ template <class _Tp, bool = is_class<_Tp>::value ||
                             is_reference<_Tp>::value ||
                             __is_nullptr_t<_Tp>::value >
 struct __libcpp_is_function
-    : public integral_constant<bool, sizeof(__libcpp_is_function_imp::__test<_Tp>(__libcpp_is_function_imp::__source<_Tp>())) == 1>
+    : public integral_constant<bool, sizeof(__libcpp_is_function_imp::__test<_Tp>(__libcpp_is_function_imp::__source<_Tp>(0))) == 1>
     {};
 template <class _Tp> struct __libcpp_is_function<_Tp, true> : public false_type {};
 
diff --git a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.cat/function.pass.cpp b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.cat/function.pass.cpp
index 82757f5035b..b1df4f2d3f4 100644
--- a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.cat/function.pass.cpp
+++ b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.cat/function.pass.cpp
@@ -13,8 +13,19 @@
 
 #include <type_traits>
 
+using namespace std;
+
+class Class {};
+
+enum Enum1 {};
+#if __cplusplus >= 201103L
+enum class Enum2 : int {};
+#else
+enum Enum2 {};
+#endif
+
 template <class T>
-void test_function_imp()
+void test()
 {
     static_assert(!std::is_void<T>::value, "");
 #if _LIBCPP_STD_VER > 11
@@ -34,19 +45,44 @@ void test_function_imp()
     static_assert( std::is_function<T>::value, "");
 }
 
-template <class T>
-void test_function()
-{
-    test_function_imp<T>();
-    test_function_imp<const T>();
-    test_function_imp<volatile T>();
-    test_function_imp<const volatile T>();
-}
+// Since we can't actually add the const volatile and ref qualifiers once
+// later let's use a macro to do it.
+#define TEST_REGULAR(...)                 \
+    test<__VA_ARGS__>();                  \
+    test<__VA_ARGS__ const>();            \
+    test<__VA_ARGS__ volatile>();         \
+    test<__VA_ARGS__ const volatile>()
+
+
+#define TEST_REF_QUALIFIED(...)           \
+    test<__VA_ARGS__ &>();                \
+    test<__VA_ARGS__ const &>();          \
+    test<__VA_ARGS__ volatile &>();       \
+    test<__VA_ARGS__ const volatile &>(); \
+    test<__VA_ARGS__ &&>();               \
+    test<__VA_ARGS__ const &&>();         \
+    test<__VA_ARGS__ volatile &&>();      \
+    test<__VA_ARGS__ const volatile &&>()
+
 
 int main()
 {
-    test_function<void ()>();
-    test_function<void (int)>();
-    test_function<int (double)>();
-    test_function<int (double, char)>();
+    TEST_REGULAR( void () );
+    TEST_REGULAR( void (int) );
+    TEST_REGULAR( int (double) );
+    TEST_REGULAR( int (double, char) );
+    TEST_REGULAR( void (...) );
+    TEST_REGULAR( void (int, ...) );
+    TEST_REGULAR( int (double, ...) );
+    TEST_REGULAR( int (double, char, ...) );
+#if __cplusplus >= 201103L
+    TEST_REF_QUALIFIED( void () );
+    TEST_REF_QUALIFIED( void (int) );
+    TEST_REF_QUALIFIED( int (double) );
+    TEST_REF_QUALIFIED( int (double, char) );
+    TEST_REF_QUALIFIED( void (...) );
+    TEST_REF_QUALIFIED( void (int, ...) );
+    TEST_REF_QUALIFIED( int (double, ...) );
+    TEST_REF_QUALIFIED( int (double, char, ...) );
+#endif
 }